Frequent [TXN OPERROR] messages

This issue has been migrated from [#4993](https://github.com/matrix-org/synapse/issues/4993).

---

In our log files appear these lines multiple times (around 5 per day). Our synapse instance is very small. Until now, I have not yet recognized problems with our synapse server which seem to be related to that. However, the log level is `WARN`, so do I need to worry?

```
16:22:32 Starting db txn 'update_presence' from sentinel context
16:22:32 Starting db connection from sentinel context: metrics will be lost
16:23:27 [TXN OPERROR] {claim_e2e_one_time_keys-19645} could not serialize access due to concurrent update
16:23:27 0/5
16:23:27 [TXN OPERROR] {claim_e2e_one_time_keys-19645} could not serialize access due to concurrent update
16:23:27 1/5
16:23:27 [TXN OPERROR] {claim_e2e_one_time_keys-19648} could not serialize access due to concurrent update
16:23:27 0/5
16:23:27 [TXN OPERROR] {add_messages_to_device_inbox-19655} could not serialize access due to concurrent update
16:23:27 0/5
16:23:27 [TXN OPERROR] {add_messages_to_device_inbox-19654} could not serialize access due to concurrent update
16:23:27 0/5
16:23:27 [TXN OPERROR] {add_messages_to_device_inbox-19652} could not serialize access due to concurrent update
16:23:27 0/5
16:23:27 [TXN OPERROR] {add_messages_to_device_inbox-19655} could not serialize access due to concurrent update
16:23:27 1/5
16:23:32 Starting db txn 'update_presence' from sentinel context
16:23:32 Starting db connection from sentinel context: metrics will be lost
```

OS: Ubuntu 18.04
Version of package `matrix-synapse-py3`: 0.99.3+bionic1
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