Store state groups separately from events (#2784)

* Split state group persist into seperate storage func

* Add per database engine code for state group id gen

* Move store_state_group to StateReadStore

This allows other workers to use it, and so resolve state.

* Hook up store_state_group

* Fix tests

* Rename _store_mult_state_groups_txn

* Rename StateGroupReadStore

* Remove redundant _have_persisted_state_group_txn

* Update comments

* Comment compute_event_context

* Set start val for state_group_id_seq

... otherwise we try to recreate old state groups

* Update comments

* Don't store state for outliers

* Update comment

* Update docstring as state groups are ints

synapse/storage/events.py

@@ -755,9 +755,8 @@ class EventsStore(SQLBaseStore):
             events_and_contexts=events_and_contexts,
         )
 
-        # Insert into the state_groups, state_groups_state, and
-        # event_to_state_groups tables.
-        self._store_mult_state_groups_txn(txn, events_and_contexts)
+        # Insert into event_to_state_groups.
+        self._store_event_state_mappings_txn(txn, events_and_contexts)
 
         # _store_rejected_events_txn filters out any events which were
         # rejected, and returns the filtered list.
@@ -992,10 +991,9 @@ class EventsStore(SQLBaseStore):
                 # an outlier in the database. We now have some state at that
                 # so we need to update the state_groups table with that state.
 
-                # insert into the state_group, state_groups_state and
-                # event_to_state_groups tables.
+                # insert into event_to_state_groups.
                 try:
-                    self._store_mult_state_groups_txn(txn, ((event, context),))
+                    self._store_event_state_mappings_txn(txn, ((event, context),))
                 except Exception:
                     logger.exception("")
                     raise