Messages hang in pending state when database connection fails

## Describe the bug

In my 3.3.4 version of Postal I have this problem that the database sometimes loses connection.

`Jul 10 08:55:44 post mariadbd[626]: 2024-07-10 6:55:44+0000 482 [Warning] Aborted connection 482 to db: 'unconnected' user: 'postal' host: 'localhost' (Got timeout reading communication packets)`

That needs to be fixed on my server... BUT Postal is not able to handle this. The message which the worker is trying to send hangs in pending state after this:

```
2024-07-10 06:55:45 +0000 DEBUG acquired task role by renewing it component=worker thread=tasks
2024-07-10 06:55:45 +0000 INFO running task component=worker thread=tasks task=SendNotificationsScheduledTask
2024-07-10 06:55:45 +0000 INFO scheduling task to next run at 2024-07-10 06:56:45 UTC component=worker thread=tasks task=SendNotificationsScheduledTask
2024-07-10 06:56:45 +0000 DEBUG acquired task role by renewing it component=worker thread=tasks
2024-07-10 06:56:45 +0000 INFO running task component=worker thread=tasks task=SendNotificationsScheduledTask
2024-07-10 06:56:45 +0000 INFO scheduling task to next run at 2024-07-10 06:57:45 UTC component=worker thread=tasks task=SendNotificationsScheduledTask
2024-07-10 06:57:03 +0000 INFO starting message unqueue component=worker thread=work0 original_queued_message=1134326
2024-07-10 06:57:03 +0000 INFO queue latency is 2.470646619796753s component=worker thread=work0 original_queued_message=1134326
2024-07-10 06:57:03 +0000 ERROR Mysql2::Error::ConnectionError (MySQL server has gone away) component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /usr/local/bundle/gems/mysql2-0.5.6/lib/mysql2/client.rb:151:in `_query' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /usr/local/bundle/gems/mysql2-0.5.6/lib/mysql2/client.rb:151:in `block in query' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /usr/local/bundle/gems/mysql2-0.5.6/lib/mysql2/client.rb:150:in `handle_interrupt' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /usr/local/bundle/gems/mysql2-0.5.6/lib/mysql2/client.rb:150:in `query' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:326:in `query_on_connection' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:318:in `block in query' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/connection_pool.rb:20:in `use' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:345:in `with_mysql' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:317:in `query' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:177:in `select' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/message.rb:12:in `find_one' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/lib/postal/message_db/database.rb:47:in `message' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/app/models/concerns/has_message.rb:12:in `message' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer/initial_processor.rb:42:in `check_message_exists' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer/initial_processor.rb:16:in `block (2 levels) in process' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer/base.rb:38:in `catch_stops' component=worker thread=work0
2024-07-10 06:57:03 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer/initial_processor.rb:14:in `block in process' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/klogger-logger-1.4.0/lib/klogger/group_set.rb:18:in `call_without_id' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/klogger-logger-1.4.0/lib/klogger/logger.rb:72:in `tagged' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer/initial_processor.rb:11:in `process' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/message_dequeuer.rb:9:in `process' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/jobs/process_queued_messages_job.rb:67:in `block in process_messages' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/relation/delegation.rb:88:in `each' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/relation/delegation.rb:88:in `each' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/jobs/process_queued_messages_job.rb:65:in `process_messages' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/jobs/process_queued_messages_job.rb:14:in `call' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:164:in `block (4 levels) in work' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/lib/ruby/3.2.0/benchmark.rb:311:in `realtime' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:164:in `block (3 levels) in work' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:285:in `capture_errors' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:161:in `block (2 levels) in work' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:160:in `each' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:160:in `block in work' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:215:in `with_connection' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:159:in `work' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:142:in `block (3 levels) in start_work_thread' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:141:in `loop' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:141:in `block (2 levels) in start_work_thread' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/klogger-logger-1.4.0/lib/klogger/group_set.rb:18:in `call_without_id' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /usr/local/bundle/gems/klogger-logger-1.4.0/lib/klogger/logger.rb:72:in `tagged' component=worker thread=work0
2024-07-10 06:57:04 +0000 ERROR /opt/postal/app/app/lib/worker/process.rb:139:in `block in start_work_thread' component=worker thread=work0
2024-07-10 06:57:06 +0000 INFO starting message unqueue component=worker thread=work1 original_queued_message=1134327
2024-07-10 06:57:06 +0000 INFO queue latency is 4.297773838043213s component=worker thread=work1 original_queued_message=1134327
2024-07-10 06:57:06 +0000 INFO found 16 associated messages to process at the same time batch_key=outgoing-gmail.com component=worker thread=work1 original_queued_message=1134327
2024-07-10 06:57:06 +0000 INFO parsing message content as it hasn't been parsed before component=worker thread=work1 original_queued_message=1134327 queued_message=1134327
2024-07-10 06:57:06 +0000 INFO Connected to log_id=Y75FMTZH component=worker thread=work1 original_queued_message=1134327 queued_message=1134327
2024-07-10 06:57:06 +0000 INFO Sending message 1::4766770 to log_id=Y75FMTZH component=worker thread=work1 original_queued_message=1134327 queued_message=1134327
2024-07-10 06:57:06 +0000 INFO Accepted by log_id=Y75FMTZH component=worker thread=work1 original_queued_message=1134327 queued_message=1134327

```

Now the message hangs in pending state because it is still locked by the failing worker process:

```
select id,message_id,locked_by,locked_at,retry_after,created_at,updated_at from queued_messages where message_id=4766769;
+---------+------------+--------------------------------------------+----------------------------+-------------+----------------------------+----------------------------+
| id | message_id | locked_by | locked_at | retry_after | created_at | updated_at |
+---------+------------+--------------------------------------------+----------------------------+-------------+----------------------------+----------------------------+
| 1134326 | 4766769 | host:mail pid:9 thread:12 22c1cddf97171ce2 | 2024-07-10 06:57:03.968458 | NULL | 2024-07-10 06:57:01.510895 | 2024-07-10 06:57:01.510895 |
+---------+------------+--------------------------------------------+----------------------------+-------------+----------------------------+----------------------------+
1 row in set (0.001 sec)

```

The only way to get it through now is setting locked_at and locked_by fields to null.

## Expected behaviour

The message should be cleaned properly by the worker so it can be picked up by another process the next time the database is back.

## Environment details

- OS: Ubuntu 22.04.4 LTS
- Version 3.3.4