Optionally return event contexts with search results

Add room context api

Implement full_state incremental sync

Fix a 500 error resulting from empty room_ids

A hopefully-complete implementation of the full_state incremental sync, as
specced at https://github.com/matrix-org/matrix-doc/pull/133.

This actually turns out to be a relatively simple modification to the initial
sync implementation.

POST /_matrix/client/api/v1/rooms//send/a.b.c gave a 500 error, because we
assumed that rooms always had at least one character.

Pull out sender when computing search results

Implement rank function for SQLite FTS

Reject events which are too large

Support filtering events represented as dicts.

Add filters to search.

This is useful because the emphemeral events such as presence and
typing are represented as dicts inside synapse.

Refactor api.filtering to have a Filter API

Cap the time to retry txns to appservices to 8.5 minutes

SPEC-222

Add config option to disable password login

Add some unit tests of prune_events()