[1.7.x] Fixed #22606 -- Locmemcache has_key() failed for infinite cache expiry

Refactored cache expiry logic for Locmemcache to make consistent across
all places where accessed, and correctly handle None as expiry time.
Backport of 66880e4cd from master.

tests/cache/tests.py

@@ -286,6 +286,8 @@ class BaseCacheTests(object):
         cache.set("hello1", "goodbye1")
         self.assertEqual(cache.has_key("hello1"), True)
         self.assertEqual(cache.has_key("goodbye1"), False)
+        cache.set("no_expiry", "here", None)
+        self.assertEqual(cache.has_key("no_expiry"), True)
 
     def test_in(self):
         # The in operator can be used to inspect cache contents