Updates to the README

Minor updates post-review

The demo client isn't really fit for purpose, so stop encouraging people to use
it.

Add some syntax highlighting

- remove redundant "where's the spec" section: this would belong in "About
matrix", but it's already there.

- E2E is in beta rather than dev

Fix a couple of broken links

Move some bits of the README around. No words were changed in the making of
this commit.

this is rst so name it accordingly

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

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

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

This only makes a difference for versions of ldap3 before 1.0, but a)
its best to be explicit and b) there are distributions that package
ancient versions for ldap3 (e.g. debian).

Limit the number of prev_events of new events

Respect use_frozen_dicts option in workers