Fix error on sqlite 3.7

Create the url_cache index on local_media_repository as a background update, so
that we can detect whether we are on sqlite or not and create a partial or
complete index accordingly.

To avoid running the cleanup job before we have built the index, add a bailout
which will defer the cleanup if the bg updates are still running.

Fixes https://github.com/matrix-org/synapse/issues/2572.

synapse/storage/background_updates.py

@@ -85,6 +85,7 @@ class BackgroundUpdateStore(SQLBaseStore):
         self._background_update_performance = {}
         self._background_update_queue = []
         self._background_update_handlers = {}
+        self._all_done = False
 
     @defer.inlineCallbacks
     def start_doing_background_updates(self):
@@ -106,8 +107,17 @@ class BackgroundUpdateStore(SQLBaseStore):
                         "No more background updates to do."
                         " Unscheduling background update task."
                     )
+                    self._all_done = True
                     defer.returnValue(None)
 
+    def has_completed_background_updates(self):
+        """Check if all the background updates have completed
+
+        Returns:
+            bool: True if all background updates have completed
+        """
+        return self._all_done
+
     @defer.inlineCallbacks
     def do_next_background_update(self, desired_duration_ms):
         """Does some amount of work on the next queued background update
@@ -269,7 +279,7 @@ class BackgroundUpdateStore(SQLBaseStore):
             # Sqlite doesn't support concurrent creation of indexes.
             #
             # We don't use partial indices on SQLite as it wasn't introduced
-            # until 3.8, and wheezy has 3.7
+            # until 3.8, and wheezy and CentOS 7 have 3.7
             #
             # We assume that sqlite doesn't give us invalid indices; however
             # we may still end up with the index existing but the