Change Cache_expire_time to a partial index because we don't need t…

…o query rows efficiently `where expire_time IS NULL` (#305)

diskcache/core.py

@@ -531,7 +531,7 @@ def __init__(self, directory=None, timeout=60, disk=Disk, **settings):
 
         sql(
             'CREATE INDEX IF NOT EXISTS Cache_expire_time ON'
-            ' Cache (expire_time)'
+            ' Cache (expire_time) WHERE expire_time IS NOT NULL'
         )
 
         query = EVICTION_POLICY[self.eviction_policy]['init']