Allow guests to access room context API

Incremental background updates for db indexes

Return world_readable and guest_can_join in /publicRooms

Allow guest users to join and message rooms

Get display name from identity server, not client

Don't fiddle with results returned by event sources

Overwriting hashes returned by other methods is poor form.

Fixes: SYN-516

The progress for each background update is stored as a JSON blob in the
database. Each background update is broken up into separate batches.
The batch size is automatically tuned to try avoid blocking single
threaded databases for too long.

... merely because I was trying to figure out how it worked, and couldn't.

SYN-513: Include updates for rooms that have had all their tags deleted

Conflicts:
	synapse/storage/tags.py

Remove accidentally added ID column

Implement pagination, order by and groups in search