We incorrectly asserted that all contexts must have a non None state
group without consider outliers. This would usually be fine as the
assertion would never be hit, as there is a shortcut during persistence
if the forward extremities don't change.

However, if the outlier is being persisted with non-outlier events, the
function would be called and the assertion would be hit.

Fixes #3601

Fix client_reader worker being able to handle /context requests

This allows us to handle /context/ requests on the client_reader worker
without having to pull in all the various stream handlers (e.g.
precence, typing, pushers etc). The only thing the token gets used for
is pagination, and that ignores everything but the room portion of the
token.

Use deltas to calculate current state deltas

If we have a delta from the existing to new current state, then we can
reuse that rather than manually working it out by fetching both lots of
state.

Improve logging for exceptions when handling PDUs

Fixes and optimisations for resolve_state_groups

Remove redundant checks on room forgottenness

Speed up _calculate_state_delta

Logcontext fixes

Make client_reader support some more read only APIs

Add some measure blocks to persist_events

Fix some random logcontext leaks.

... to help us figure out where 40% of CPU is going

when we get an exception handling a federation PDU, log the whole stacktrace.

it's easier to create the new state group as a delta from the existing one.

(There's an outside chance this will help with
https://github.com/matrix-org/synapse/issues/3364)