explicitly close keys db in saveState

Should be redundant, but test suite is ending up with
a lot of extra sqlite connections before unused keys database handles
get garbage collected.

While running the test suite, I often saw 2-4+ open fds to the same
repo's keys database. After this change, it seems to mostly have 1,
occasionally 2.

And that might explain some of the strange sqlite failures in the test suite.
Especially the failures of test_lock_v7_force, where the keys database
gets renamed to a new directory out from under sqlite.

Annex/Content.hs

@@ -867,6 +867,7 @@ getKeysPresent keyloc = do
 saveState :: Bool -> Annex ()
 saveState nocommit = doSideAction $ do
 	Annex.Queue.flush
+	Database.Keys.closeDb
 	unless nocommit $
 		whenM (annexAlwaysCommit <$> Annex.getGitConfig) $
 			Annex.Branch.commit =<< Annex.Branch.commitMessage