Add some measure blocks to persist_events

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

Only get cached state from context in persist_event

We don't want to bother pulling out the current state from the DB since
until we know we have to. Checking the context for state is just an
optimisation.

Fix missing attributes on workers.

This was missed during the transition from attribute to getter for
getting state from context.

Fix EventContext when using workers

We were:
  1. Not correctly setting all attributes
  2. Using defer.inlineCallbacks in a non-generator

Add concept of StatelessContext, take 4.

Refcator EventContext to accept state during init

Announce deleted devices explicitly over federation.

Test and fix support for cancellation in Linearizer

A set of improvements to the Limiter

Linearizer was effectively a Limiter with max_count=1, so rather than
maintaining two sets of code, let's combine them.

* give them names, to improve logging
* use a deque rather than a list for efficiency

Fixes #3570

Switch changes to markdown & flip content on for miscs