Currently the HTTP handlers are attached to an API mux which is created inside
Setup(). In the future, we'll want to split this out so a single mux can have
both sync and clientapi handlers attached to it.

This involves:
 - Parsing the HTTP request
 - Requesting auth events from the roomserver via the Query API
 - Building the event
 - Doing auth checks on the event
 - Sending it to the roomserver input log