Shuffle persist event data store functions. (#7440)

The aim here is to get to a stage where we have a `PersistEventStore` that holds all the write methods used during event persistence, so that we can take that class out of the `DataStore` mixin and instansiate it separately. This will allow us to instansiate it on processes other than master, while also ensuring it is only available on processes that are configured to write to events stream.

This is a bit of an architectural change, where we end up with multiple classes per data store (rather than one per data store we have now). We end up having:

1. Storage classes that provide high level APIs that can talk to multiple data stores.
2. Data store modules that consist of classes that must point at the same database instance.
3. Classes in a data store that can be instantiated on processes depending on config.

synapse/storage/data_stores/main/state.py

@@ -16,17 +16,12 @@
 import collections.abc
 import logging
 from collections import namedtuple
-from typing import Iterable, Tuple
-
-from six import iteritems
 
 from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import NotFoundError, UnsupportedRoomVersionError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
-from synapse.events import EventBase
-from synapse.events.snapshot import EventContext
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.data_stores.main.events_worker import EventsWorkerStore
 from synapse.storage.data_stores.main.roommember import RoomMemberWorkerStore
@@ -473,33 +468,3 @@ class StateStore(StateGroupWorkerStore, MainStateBackgroundUpdateStore):
 
     def __init__(self, database: Database, db_conn, hs):
         super(StateStore, self).__init__(database, db_conn, hs)
-
-    def _store_event_state_mappings_txn(
-        self, txn, events_and_contexts: Iterable[Tuple[EventBase, EventContext]]
-    ):
-        state_groups = {}
-        for event, context in events_and_contexts:
-            if event.internal_metadata.is_outlier():
-                continue
-
-            # if the event was rejected, just give it the same state as its
-            # predecessor.
-            if context.rejected:
-                state_groups[event.event_id] = context.state_group_before_event
-                continue
-
-            state_groups[event.event_id] = context.state_group
-
-        self.db.simple_insert_many_txn(
-            txn,
-            table="event_to_state_groups",
-            values=[
-                {"state_group": state_group_id, "event_id": event_id}
-                for event_id, state_group_id in iteritems(state_groups)
-            ],
-        )
-
-        for event_id, state_group_id in iteritems(state_groups):
-            txn.call_after(
-                self._get_state_group_for_event.prefill, (event_id,), state_group_id
-            )