Make the spam checker a module

Sometimes it's a Mock object which is not none but is still not
what we're after

Factor out module loading to a separate place

So it can be reused

Refactor to speed up incremental syncs

Clarify recommended network setup

Fix iteration of requests_missing_keys; list doesn't have .values()

preserve_context_over_function doesn't do what you want it to do.

Support SRV records which point at AAAA records, as well as A records.

Fixes https://github.com/matrix-org/synapse/issues/2405

Clean up Keyring code

preserve_context_over_fn is essentially broken, because (a) it pointlessly
drops the current logcontext before calling its wrapped function, which means
we don't get any useful logcontexts for _handle_key_deferred; (b) it wraps the
resulting deferred in a _PreservingContextDeferred, which is very dangerous
because you then can't yield on it without leaking context back into the
reactor.

Instead, let's specify that the resultant deferreds call their callbacks with
no logcontext.

... which means that logcontexts can be correctly preserved for the stuff it
does.

get_server_verify_keys is now called with the logcontext, so needs to
preserve_fn when it fires off its nested inlineCallbacks function.

Also renames get_server_verify_keys to reflect the fact it's meant to be
private.

If the verify_request.deferred has already completed, then `remove_deferreds`
will be called immediately. It therefore might resolve the server_to_deferred
deferred while there are still other requests for that server in flight.

To avoid that, we should build the complete list of requests, and *then* add the
callbacks.

Define that it is run with no log context, and make sure that happens.

If we aren't careful to reset the logcontext, we can't bung the deferreds into
defer.gatherResults etc. We don't actually do that directly, but we *do*
resolve other deferreds from affected callbacks (notably the server_to_deferred
map in _start_key_lookups), and those *do* get passed into
defer.gatherResults. It turns out that this way ends up being least confusing.

... to make it easier to see what's going on.

This is a precursor to factoring some of this code out.

There's no need for this to be a nested definition; pulling it out not only
makes it more efficient, but makes it easier to check that it's not accessing
any local variables it shouldn't be.

This might make the cache slightly more efficient.