fix some syntax errors for user search when search_all_users is enabled

fixes #2801, hopefully

Make it possible to run tests against postgres

... instead of creating our own special SQLiteMemoryDbPool, whose purpose was a
bit of a mystery.

For some reason this makes one of the tests run slightly slower, so bump the
sleep(). Sorry.

Configure the connectionpool used for unit tests to run the `on_new_connection`
function.

Remove unused/bitrotted MemoryDataStore

Factor out get_db_conn to HomeServer base class

Fix server 500 on public rooms call when no rooms exist

This function is identical to all subclasses, so we may as well push it up to
the base class to reduce duplication (and make use of it in the tests)

This isn't used, and looks thoroughly bitrotted.

rather than try reconstruct the results object, better to guard against the xrange step argument being 0

synapse 500s on a call to publicRooms in the case where the number of public rooms is zero, the specific cause is due to xrange trying to use a step value of zero, but if the total room number really is zero then it makes sense to just bail and save the extra processing

Add some comments about the reactor tick time metric

blindly implement ?ts for AS. untested

Matthew's fixes to the unit tests

Add federation_domain_whitelist

gives a way to restrict which domains your HS is allowed to federate with.
useful mainly for gracefully preventing a private but internet-connected HS from trying to federate to the wider public Matrix network

Extracted from https://github.com/matrix-org/synapse/pull/2820

add registrations_require_3pid and allow_local_3pids

Use the right path for url_preview thumbnails

This was introduced by #2627: we were overwriting the original media for url
previews with the thumbnails :/

(fixes https://github.com/vector-im/riot-web/issues/6012, hopefully)

 * [ ] 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

Make storage providers configurable