Make email notifs work on the pusher synapse

Add a slaved datastore for account data

Allow receipts for events we haven't seen in the db

Create user with expiry

Fixup add_pusher

Delete old pushers

Use tree cache for get_linearized_receipts_for_room

Remove unused indices

 - Add unittests for client, api and handler

Signed-off-by: Negar Fazeli <negar.fazeli@ericsson.com>

This includes removing both unused indices and indices that are subsets
of other indices.

Fix a bug in replication that was causing the pusher to tight loop

Plus general bugfix to email notif code

move filter_events_for_client out of base handler

Shuffle when we get the signing_key attribute.

Wait until we sign a message to get the signing key from the homeserver
config. This means that the message handler can be created without
having a signing key in the config which means that separate processes
like the pusher that don't send messages and don't need to sign them can
still access the handlers.

Correctly handle NULL password hashes from the database

Shuffle some methods out of base handler