Add source interface for getting the cache ltime

In order to save a list of ltimes for each connection we need a source
interface to get the current ltime of a project branch cache.

Change-Id: If01db0698024beeed813d2c9910651c757377865

zuul/source/__init__.py

@@ -125,6 +125,10 @@ class BaseSource(object, metaclass=abc.ABCMeta):
 
         """
 
+    @abc.abstractmethod
+    def getProjectBranchCacheLtime(self):
+        """Return the current ltime of the project branch cache."""
+
     @abc.abstractmethod
     def getRequireFilters(self, config):
         """Return a list of ChangeFilters for the scheduler to match against.