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.

Command/Smudge.hs

@@ -16,7 +16,7 @@ import Annex.FileMatcher
 import Types.KeySource
 import Backend
 import Logs.Location
-import qualified Database.AssociatedFiles as AssociatedFiles
+import qualified Database.Keys
 
 import qualified Data.ByteString.Lazy as B
 
@@ -103,5 +103,5 @@ emitPointer = putStrLn . formatPointer
 
 updateAssociatedFiles :: Key -> FilePath -> Annex ()
 updateAssociatedFiles k f = do
-	AssociatedFiles.addDb k f
-	AssociatedFiles.flushDb
+	Database.Keys.addAssociatedFile k f
+	Database.Keys.flushDb