Feature request: if Electric can't connect to Postgres due to long-running transactions, query and log what the transactions are

**Is your feature request related to a problem? Please describe.**
An Electric Cloud customer ran into an issue where Electric couldn't create a publication slot due to a long-running transaction in PG. They (and we) didn't know what the problem was for a while so they had downtime while diagnosing the problem.

**Describe the solution you'd like**
In addition to Electric logging it can't connect, it should query for what the long-running txs are to help the user resolve things e.g. put a shorter limit on `idle_in_transaction_session_timeout`.

**Additional context**
Some possible queries:

Shows active transactions sorted by tx length:

```sql
SELECT
pid,
usename,
application_name,
state,
age(now(), xact_start) AS xact_age,
query
FROM
pg_stat_activity
WHERE
xact_start IS NOT NULL
ORDER BY
xact_age DESC;
```

Shows idle transactions:

```sql
SELECT
pid,
usename,
application_name,
client_addr,
state,
query_start,
xact_start,
age(now(), xact_start) AS xact_age,
query
FROM
pg_stat_activity
WHERE
state = 'idle in transaction'
ORDER BY
xact_age DESC;
```