This is in preparation for moving GET /context/ to a worker

This will let us call the read only parts from workers, and so be able
to move some APIs off of master, e.g. the `/state` API.

Remove event re-signing hacks

Comment dummy TURN parameters in default config

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