Insertion of messages in capped collection is slow

[odesenfans]

Running some benchmarks on DB insertion time, I noticed that the insertion time in the CappedMessage collection is two magnitudes of order slower than insertions in the Message collection.

2022-05-19 11:59:33 [INFO] root: Wrote 880 documents to collection Message in 0.5942 seconds
2022-05-19 11:59:33 [INFO] root: Wrote 880 documents to collection PendingMessage in 0.1174 seconds
2022-05-19 11:59:41 [INFO] root: Wrote 880 documents to CappedMessage in 7.9019 seconds

This slows down the message processing job quite a bit. Given that capped collections have other painful limitations (ex: no updates that increase the size, e.g. for confirmations), we should investigate whether this capped collection can be dropped in favor of a better solution for the websocket.

An aggregate could be a good solution. Ex: aggregate([{$sort: {"reception_time": -1}}, {$limit: 5000}, {$match: <query-filters>}]) using the future reception_time field introduced in #267.

[odesenfans]

Replaced the DB, no more capped collections.