This fixes #3518, and ensures that we get useful logs and metrics for lots of
things that happen in the background.

(There are certainly more things that happen in the background; these are just
the common ones I've found running a single-process synapse locally).

Resource tracking for background processes

Remove event re-signing hacks

Comment dummy TURN parameters in default config

This introduces a mechanism for tracking resource usage by background
processes, along with an example of how it will be used.

This will help address #3518, but more importantly will give us better insights
into things which are happening but not being shown up by the request metrics.

We *could* do this with Measure blocks, but:
 - I think having them pulled out as a completely separate metric class will
   make it easier to distinguish top-level processes from those which are
   nested.

 - I want to be able to report on in-flight background processes, and I don't
   think we want to do this for *all* Measure blocks.

Fix visibility of events from erased users over federation

Refactor and optimze filter_events_for_server

Fix perf regression in PR #3530

The get_entities_changed function was changed to return all changed
entities since the given stream position, rather than only those changed
from a given list of entities. This resulted in the function incorrectly
returning large numbers of entities that, for example, caused large
increases in database usage.

Don't return unknown entities in get_entities_changed

check isort by travis

for easier unit testing.

Use parse and asserts from http.servlet

the method "assert_params_in_request" does handle dicts and not
requests. A request body has to be parsed to json before this method
can be used

parse_integer and parse_string can take a request and raise errors
in case we have wrong or missing params.
This PR tries to use them more to deduplicate some code and make it
better readable

Make FederationRateLimiter queue requests properly