Separate get_current_token into two. (#8113)

The function is used for two purposes: 1) for subscribers of streams to
get a token they can use to get further updates with, and 2) for
replication to track position of the writers of the stream.

For streams with a single writer the two scenarios produce the same
result, however the situation becomes complicated for streams with
multiple writers. The current `MultiWriterIdGenerator` does not
correctly handle the first case (which is not an issue as its only used
for the `caches` stream which nothing subscribes to outside of
replication).

synapse/replication/slave/storage/_slaved_id_tracker.py

@@ -33,3 +33,11 @@ class SlavedIdTracker(object):
             int
         """
         return self._current
+
+    def get_current_token_for_writer(self, instance_name: str) -> int:
+        """Returns the position of the given writer.
+
+        For streams with single writers this is equivalent to
+        `get_current_token`.
+        """
+        return self.get_current_token()

synapse/replication/tcp/streams/_base.py

@@ -405,7 +405,7 @@ class CachesStream(Stream):
         store = hs.get_datastore()
         super().__init__(
             hs.get_instance_name(),
-            store.get_cache_stream_token,
+            store.get_cache_stream_token_for_writer,
             store.get_all_updated_caches,
         )