Cache branches in connections/sources

The current attempt to caches branches is ineffective -- we
query the list of branches during every tenant reconfiguration.

The list of branches for a project is really global information;
we might cache it on the Abide, however, drivers may need to filter
that list based on tenant configuration (eg, github protected
branches).  To accomodate that, just allow/expect the drivers to
perform their own caching of branches, and to generally keep
the list up to date (or at least invalidate their caches) by
observing branch create/delete events.

A full reconfiguration instructs the connections to clear their
caches so that we perform a full query.  That way, an operator
can correct from a situation where the cache is invalid.

Change-Id: I3bd0cda5875dd21368e384e3704a61ebb5dcedfa

zuul/connection/__init__.py

@@ -68,12 +68,25 @@ class BaseConnection(object, metaclass=abc.ABCMeta):
     def registerScheduler(self, sched):
         self.sched = sched
 
+    def clearBranchCache(self):
+        """Clear the branch cache for this connection.
+
+        This is called immediately prior to performing a full
+        reconfiguration.  The branch cache should be cleared so that a
+        full reconfiguration can be used to correct any errors in
+        cached data.
+
+        """
+        pass
+
     def maintainCache(self, relevant):
+
         """Make cache contain relevant changes.
 
         This lets the user supply a list of change objects that are
         still in use.  Anything in our cache that isn't in the supplied
         list should be safe to remove from the cache."""
+        pass
 
     def getWebController(self, zuul_web):
         """Return a cherrypy web controller to register with zuul-web.