Factor out `SrvResolver`

[DMRobertson]

Sydent has an SrvResolver class here: https://github.com/matrix-org/sydent/blob/8331f94e1e22c72c84683c271142f3237c707a27/sydent/http/srvresolver.py#L104

Synapse has one too with an associated test case.

I couldn't see one in Sygnal.

[DMRobertson]

I think the entire srv_resolver file is duplicated in both cases.

[clokep]

I think the entire srv_resolver file is duplicated in both cases.

I believe this is the case (or meant to be the case). 😄

[DMRobertson]

I looked at this briefly today, but was dismayed to see that Synapse's implementation includes make_deferred_awaitable which exists to interact with the logcontext machinery.

We could pull the log context into matrix python common. I would advocate pretty strongly against that. That feels like inappropriate coupling between "make this HTTP request" and "oh, by the way we are tracking CPU time".

[DMRobertson]

See also matrix-org/synapse#10342.