Remove event fetching from DB threads

[erikjohnston]

No description provided.

[NegativeMjark]

LGTM

[ara4n]

what's the rationale here?

[erikjohnston]

@ara4n:

what's the rationale here?

Mainly consistency:

1. Consistency with everywhere else ─ most of these are uncommon code paths.
2. It means we can get rid of the horrible duplication between the txn and non-txn versions of fetching events (turns out its not easy to split out the common code)
3. Everything now goes through the same bulk fetching of events code, which is nice.
4. The event cache no longer needs to be thread safe.
5. The DB metrics report how long the actual queries take, rather than also including the time it takes to get events.
6. Fetching events takes a bit of CPU time, and I'd rather do CPU work on the main thread, as otherwise it takes up a DB thread (which are in relatively limited supply)