Remove broken use of clock.call_later

background_updates was using `call_later` in a way that leaked the logcontext
into the reactor.

We could have rewritten it to do it properly, but given that we weren't using
the fancier facilities provided by `call_later`, we might as well just use
`async.sleep`, which does the logcontext stuff properly.

synapse/storage/background_updates.py

@@ -12,6 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import synapse.util.async
 
 from ._base import SQLBaseStore
 from . import engines
@@ -84,24 +85,14 @@ class BackgroundUpdateStore(SQLBaseStore):
         self._background_update_performance = {}
         self._background_update_queue = []
         self._background_update_handlers = {}
-        self._background_update_timer = None
 
     @defer.inlineCallbacks
     def start_doing_background_updates(self):
-        assert self._background_update_timer is None, \
-            "background updates already running"
-
         logger.info("Starting background schema updates")
 
         while True:
-            sleep = defer.Deferred()
-            self._background_update_timer = self._clock.call_later(
-                self.BACKGROUND_UPDATE_INTERVAL_MS / 1000., sleep.callback, None
-            )
-            try:
-                yield sleep
-            finally:
-                self._background_update_timer = None
+            yield synapse.util.async.sleep(
+                self.BACKGROUND_UPDATE_INTERVAL_MS / 1000.)
 
             try:
                 result = yield self.do_next_background_update(