upgrade: Handle upgrade to v6 when the repository already contains v6 unlocked files whose content is already present.

Closes https://github.com/datalad/datalad/issues/1020

The use of runWriter in scanUnlockedFiles broke due to this change;
it failed with blocked indefinitely in mvar, because the database write
handle was taken while linkFromAnnex needed to also write to it (to update
the inode cache). So, switched to using a separate runWriter for each call
to addAssociatedFileFast. A little less efficient, but not greatly; the
writes should all still be cached.

Command/Unlock.hs

@@ -56,7 +56,7 @@ performNew dest key = do
 				case r of
 					LinkAnnexOk -> return ()
 					LinkAnnexNoop -> return ()
-					_ -> error "unlock failed"
+					LinkAnnexFailed -> error "unlock failed"
 			, liftIO $ writePointerFile tmp key destmode
 			)
 	next $ cleanupNew dest key destmode