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

(mostly just clarifications)