Fix some instances of ExpiringCache not expiring cache items

ExpiringCache required that `start()` be called before it would actually
start expiring entries. A number of places didn't do that.

This PR removes `start` from ExpiringCache, and automatically starts
backround reaping process on creation instead.

synapse/util/caches/expiringcache.py

@@ -58,7 +58,6 @@ class ExpiringCache(object):
 
         self.metrics = register_cache("expiring", cache_name, self)
 
-    def start(self):
         if not self._expiry_ms:
             # Don't bother starting the loop if things never expire
             return