Fix a logcontext leak in persist_events

Metrics for events processed in appservice and fed sender

Metrics for number of RDATA commands received

ObserveableDeferred expects its callbacks to be called without any
logcontexts, whereas it turns out we were calling them with the logcontext of
the request which initiated the persistence loop.

It seems wrong that we are attributing work done in the persistence loop to the
request that happened to initiate it, so let's solve this by dropping the
logcontext for it.

(I'm not sure this actually causes any real problems other than messages in the
debug log, but let's clean it up anyway)

Refactor MediaRepository to separate out storage

More metrics I wished I'd had

I found myself wishing we had this.

Make Counter render floats

Prometheus handles all metrics as floats, and sometimes we store non-integer
values in them (notably, durations in seconds), so let's render them as floats
too.

(Note that the standard client libraries also treat Counters as floats.)

Reinstate media download on thumbnail request

We need to actually download the remote media when we get a request for a
thumbnail.

When using synctl with workers, don't start the main synapse automatically

Add /room/{id}/event/{id} to synapse

Do bcrypt hashing in a background thread

Fix publicised groups GET API (singular) over federation

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

which was missing its fed client API, since there is no other API
it might as well reuse the bulk one and unwrap it

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

Update http request metrics before calling servlet

Make sure that we set the servlet name in the metrics object *before* calling
the servlet, in case the servlet throws an exception.