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.

tests/util/test_expiring_cache.py

@@ -65,7 +65,6 @@ class ExpiringCacheTestCase(unittest.TestCase):
     def test_time_eviction(self):
         clock = MockClock()
         cache = ExpiringCache("test", clock, expiry_ms=1000)
-        cache.start()
 
         cache["key"] = 1
         clock.advance_time(0.5)