Refactor MediaRepository to separate out storage

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.