Revise logging output

*This issue was originally created by [**@neilalexander**](https://github.com/neilalexander) at .*

We need to comb through the various places that we produce log output and adjust the levels and such. It’s virtually impossible for people running a Dendrite homeserver to diagnose problems or to provide logs for issues because:

1. There’s just so much noise across all the levels, including a lot at `level=info` which could be `debug`/`trace`, much of it not all that useful for day-to-day use;
2. A lot of things are listed as `level=error` that really should be `level=warn`, e.g. when we can’t talk to specific servers, things that can time out or hit a context deadline;
3. There’s obvious gaps in the errors produced as well, like join responses don’t seem to be well logged or easy to grep for at all;
4. Some log entries don’t contain useful context, i.e. they contain event IDs but not room IDs or other such mistakes.

Let’s do something about that so that the logs are actually useful and easy to understand.