Return early on /sync code paths if a '*' filter is used

This is currently very conservative in that it only does this if there is no
`since` token. This limits the risk to clients likely to be doing one-off
syncs (like bridges), but does mean that normal human clients won't benefit
from the time savings here. If the savings are large enough, I would consider
generalising this to just check the filter.

Implement "event_fields" in filters

Merge branch 'dbkr/work_around_devicename_bug' of github.com:matrix-org/synapse into release-v0.18.4

Also make it an inclusive not exclusive filter, as the spec demands.

Fail with a coherent error message if `/sync?filter=` is invalid

Works around https://github.com/vector-im/vector-android/issues/715
and equivalent for iOS

Use external ldap auth pacakge

Correctly intern keys in state cache

Store Promise<Response> instead of Response for HTTP API transactions

Add support for durations in minutes

This fixes a race whereby:
 - User hits an endpoint.
 - No cached transaction so executes main code.
 - User hits same endpoint.
 - No cache transaction so executes main code.
 - Main code finishes executing and caches response and returns.
 - Main code finishes executing and caches response and returns.

 This race is common in the wild when Synapse is struggling under load.
 This commit fixes the race by:
  - User hits an endpoint.
  - Caches the promise to execute the main code and executes main code.
  - User hits same endpoint.
  - Yields on the same promise as the first request.
  - Main code finishes executing and returns, unblocking both requests.

Don't assume providers raise ConfigError's