add inode cache to the db

Renamed the db to keys, since it is various info about a Keys.

Dropping a key will update its pointer files, as long as their content can
be verified to be unmodified. This falls back to checksum verification, but
I want it to use an InodeCache of the key, for speed. But, I have not made
anything populate that cache yet.

Annex/Content/Direct.hs

@@ -180,15 +180,6 @@ sameFileStatus key f status = do
 		([], Nothing) -> return True
 		_ -> return False
 
-{- If the inodes have changed, only the size and mtime are compared. -}
-compareInodeCaches :: InodeCache -> InodeCache -> Annex Bool
-compareInodeCaches x y
-	| compareStrong x y = return True
-	| otherwise = ifM inodesChanged
-		( return $ compareWeak x y
-		, return False
-		)
-
 elemInodeCaches :: InodeCache -> [InodeCache] -> Annex Bool
 elemInodeCaches _ [] = return False
 elemInodeCaches c (l:ls) = ifM (compareInodeCaches c l)