* [ ] split config options into allowed_local_3pids and registrations_require_3pid
 * [ ] simplify and comment logic for picking registration flows
 * [ ] fix docstring and move check_3pid_allowed into a new util module
 * [ ] use check_3pid_allowed everywhere

@erikjohnston PTAL

lets homeservers specify a whitelist for 3PIDs that users are allowed to associate with.
Typically useful for stopping people from registering with non-work emails

Log room when doing state resolution

Mostly because it helps figure out what is prompting the resolution

Keep track of last access time for local media

fix SQL when searching all users

Split resolve_events into two functions

... so that the return type doesn't depend on the arg types

Track DB scheduling delay per-request

Sanity checking for user ids

Check the user_id passed to a couple of APIs for validity, to avoid
"IndexError: list index out of range" exception which looks scary and results
in a 500 rather than a more useful error.

Fixes #1432, among other things

Track db txn time in millisecs

Fix 'NoneType' object has no attribute 'writeHeaders'

rework runInteraction in terms of runConnection

Optimise LoggingContext creation and copying

Fix thumbnailing remote files

Avoid throwing a (harmless) exception when we try to write an error response to
an http request where the client has disconnected.

This comes up as a CRITICAL error in the logs which tends to mislead people
into thinking there's an actual problem

For each request, track the amount of time spent waiting for a db
connection. This entails adding it to the LoggingContext and we may as well add
metrics for it while we are passing.

... so that we can share the code

... to reduce the amount of floating-point foo we do.

It turns out that the only thing we use the __dict__ of LoggingContext for is
`request`, and given we create lots of LoggingContexts and then copy them every
time we do a db transaction or log line, using the __dict__ seems a bit
redundant. Let's try to optimise things by making the request attribute
explicit.