Message buffer rework

[boreq]

Message buffer is far from perfect right now.

• it should ensure that a failure to update one feed doesn't mean that other feeds can't be updated
• if some messages are missing and therefore can't be appended to a feed it should keep messages for a specific amount of time as messages can arrive out of order and try to reorder them (needed for Decouple domain and application by using pubsub #45)
• it should try to delay validating signatures on messages and just peek their sequence numbers in case we get the same message from multiple sources (probably should replace the mechanism which lets the replication code get the sequence number from the message buffer, this is not really reliable if messages arrive out of order)

[boreq]

This was partially done but let's keep this open as we need more fixes here.

[boreq]

Closing as this is one of those long-running issues that will never be closed. We need to create small tasks for further smaller fixes in the future.