Prevent join->join membership transitions changing member count (#7977)

`StatsHandler` handles updates to the `current_state_delta_stream`, and updates room stats such as the amount of state events, joined users, etc.

However, it counts every new join membership as a new user entering a room (and that user being in another room), whereas it's possible for a user's membership status to go from join -> join, for instance when they change their per-room profile information.

This PR adds a check for join->join membership transitions, and bails out early, as none of the further checks are necessary at that point.

Due to this bug, membership stats in many rooms have ended up being wildly larger than their true values. I am not sure if we also want to include a migration step which recalculates these statistics (possibly using the `_populate_stats_process_rooms` bg update).

Bug introduced in the initial implementation https://github.com/matrix-org/synapse/pull/4338.

tests/rest/client/v1/utils.py

@@ -88,7 +88,28 @@ class RestHelper(object):
             expect_code=expect_code,
         )
 
-    def change_membership(self, room, src, targ, membership, tok=None, expect_code=200):
+    def change_membership(
+        self,
+        room: str,
+        src: str,
+        targ: str,
+        membership: str,
+        extra_data: dict = {},
+        tok: Optional[str] = None,
+        expect_code: int = 200,
+    ) -> None:
+        """
+        Send a membership state event into a room.
+
+        Args:
+            room: The ID of the room to send to
+            src: The mxid of the event sender
+            targ: The mxid of the event's target. The state key
+            membership: The type of membership event
+            extra_data: Extra information to include in the content of the event
+            tok: The user access token to use
+            expect_code: The expected HTTP response code
+        """
         temp_id = self.auth_user_id
         self.auth_user_id = src
 
@@ -97,6 +118,7 @@ class RestHelper(object):
             path = path + "?access_token=%s" % tok
 
         data = {"membership": membership}
+        data.update(extra_data)
 
         request, channel = make_request(
             self.hs.get_reactor(), "PUT", path, json.dumps(data).encode("utf8")