Neilj/fix mau initial reserved users (#4211)

* fix transaction wrapping bug that caused get_user_id_by_threepid_txn to fail

* towncrier

* white space

changelog.d/4211.bugfix

@@ -0,0 +1,2 @@
+fix start up failure when mau_limit_reserved_threepids set and db is postgres 
+

synapse/storage/monthly_active_users.py

@@ -34,8 +34,9 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         self.hs = hs
         self.reserved_users = ()
         # Do not add more reserved users than the total allowable number
-        self._initialise_reserved_users(
-            dbconn.cursor(),
+        self._new_transaction(
+            dbconn, "initialise_mau_threepids", [], [],
+            self._initialise_reserved_users,
             hs.config.mau_limits_reserved_threepids[:self.hs.config.max_mau_value],
         )