Add function to load config without generating it

Renames ``load_config`` to ``load_or_generate_config``
Adds a method called ``load_config`` that just loads the
config.

The main synapse.app.homeserver will continue to use
``load_or_generate_config`` to retain backwards compat.
However new worker processes can use ``load_config`` to
load the config avoiding some of the cruft needed to generate
the config.

As the new ``load_config`` method is expected to be used by new
configs it removes support for the legacy commandline overrides
that ``load_or_generate_config`` supports

502 on /thumbnail when can't contact remote server

Fix a bug caused by a change in auth_handler function

Remove redundant exception log in /events

Fix the relevant unit test cases

Pull full state for each room all at once

Don't make rooms visibile by default

* Disable the synchrotron on jenkins until the sytest support lands

* Poke jenkins

* Poke jenkins

* Poke jenkins

* Poke jenkins

* Poke jenkins

* Poke jenkins

* Poke jenkins

* Poke jenkins

Enable auth on /publicRoom endpoints

Add some logging for when servers ask for missing events

Don't hit DB for noop replications queries

Add gc_threshold to pusher and synchrotron

Add GC counts to metrics

Add script for running sytest with dendron

Allow setting of gc.set_thresholds

Remove dead code.

Loading push rules now happens in the datastore, so we can remove
the methods that loaded them outside the datastore.

The ``waiting_for_join_list`` in federation handler is populated by
anything, so can be removed.

The ``_get_members_events_txn`` method isn't called from anywhere
so can be removed.