Add some metrics on notifier

Phone number registration / login support v2

Fix routing loop when fetching remote media

Reduce number of spurious sync result generations.

Speed up public room list

- don't blindly proxy all HTTPRequestExceptions
- log unexpected exceptions at error
- avoid `isinstance`
- improve docs on `from_http_response_exception`

Give CodeMessageException back its `msg` attribute, and use that to hold the
HTTP status message for HttpResponseException.

Fix current_state_events table to not lie

If we try and persist two state events that have the same ancestor we
calculate the wrong current state when persisting those events.

Implement delete_devices API

(But this doesn't implement the same for deleting access tokens or e2e keys.

Also respond to code review.

Only offer msisdn flows if the x_show_msisdn option is given.

Changes from https://github.com/matrix-org/synapse/pull/1971

Add helpful texts to logger config options

This implements the proposal here https://docs.google.com/document/d/1C-25Gqz3TXy2jIAoeOKxpNtmme0jI4g3yFGqv5GlAAk for deleting multiple devices at once in a single request.

When we proxy a media request to a remote server, add a query-param, which will
tell the remote server to 404 if it doesn't recognise the server_name.

This should fix a routing loop where the server keeps forwarding back to
itself.

Also improves the error handling on remote media fetches, so that we don't
always return a rather obscure 502.

Refactor FederationServer._handle_new_pdu

Add an option to disable stdio redirect

Reread log config on SIGHUP

This makes it tractable to run synapse under pdb.

- to make it easier to add more config options.

When we are using a log_config file, reread it on SIGHUP.

Noop repated delete device inbox calls from /sync