Add EventStreamPosition type (#8388)

The idea is to remove some of the places we pass around `int`, where it can represent one of two things:

1. the position of an event in the stream; or
2. a token that partitions the stream, used as part of the stream tokens.

The valid operations are then:

1. did a position happen before or after a token;
2. get all events that happened before or after a token; and
3. get all events between two tokens.

(Note that we don't want to allow other operations as we want to change the tokens to be vector clocks rather than simple ints)

tests/replication/slave/storage/test_events.py

@@ -20,6 +20,7 @@ from synapse.events import FrozenEvent, _EventInternalMetadata, make_event_from_
 from synapse.handlers.room import RoomEventSource
 from synapse.replication.slave.storage.events import SlavedEventStore
 from synapse.storage.roommember import RoomsForUser
+from synapse.types import PersistedEventPosition
 
 from tests.server import FakeTransport
 
@@ -204,10 +205,14 @@ class SlavedEventStoreTestCase(BaseSlavedStoreTestCase):
             type="m.room.member", sender=USER_ID_2, key=USER_ID_2, membership="join"
         )
         self.replicate()
+
+        expected_pos = PersistedEventPosition(
+            "master", j2.internal_metadata.stream_ordering
+        )
         self.check(
             "get_rooms_for_user_with_stream_ordering",
             (USER_ID_2,),
-            {(ROOM_ID, j2.internal_metadata.stream_ordering)},
+            {(ROOM_ID, expected_pos)},
         )
 
     def test_get_rooms_for_user_with_stream_ordering_with_multi_event_persist(self):
@@ -293,9 +298,10 @@ class SlavedEventStoreTestCase(BaseSlavedStoreTestCase):
             # the membership change is only any use to us if the room is in the
             # joined_rooms list.
             if membership_changes:
-                self.assertEqual(
-                    joined_rooms, {(ROOM_ID, j2.internal_metadata.stream_ordering)}
+                expected_pos = PersistedEventPosition(
+                    "master", j2.internal_metadata.stream_ordering
                 )
+                self.assertEqual(joined_rooms, {(ROOM_ID, expected_pos)})
 
     event_id = 0