fix a thundering herd problem

Avoid repeatedly opening keys db when accessing a local git remote and -J
is used.

What was happening was that Remote.Git.onLocal created a new annex state
as each thread started up. The way the MVar was used did not prevent that.
And that, in turn, led to repeated opening of the keys db, as well as
probably other extra work or resource use.

Also managed to get rid of Annex.remoteannexstate, and it turned out there
was an unncessary Maybe in the keysdbhandle, since the handle starts out
closed.

Database/Keys.hs

@@ -61,7 +61,7 @@ import qualified System.FilePath.ByteString as P
  -}
 runReader :: Monoid v => (SQL.ReadHandle -> Annex v) -> Annex v
 runReader a = do
-	h <- getDbHandle
+	h <- Annex.getState Annex.keysdbhandle
 	withDbState h go
   where
 	go DbUnavailable = return (mempty, DbUnavailable)
@@ -85,7 +85,7 @@ runReaderIO a = runReader (liftIO . a)
  - The database is created if it doesn't exist yet. -}
 runWriter :: (SQL.WriteHandle -> Annex ()) -> Annex ()
 runWriter a = do
-	h <- getDbHandle
+	h <- Annex.getState Annex.keysdbhandle
 	withDbState h go
   where
 	go st@(DbOpen qh) = do
@@ -101,17 +101,6 @@ runWriter a = do
 runWriterIO :: (SQL.WriteHandle -> IO ()) -> Annex ()
 runWriterIO a = runWriter (liftIO . a)
 
-{- Gets the handle cached in Annex state; creates a new one if it's not yet
- - available, but doesn't open the database. -}
-getDbHandle :: Annex DbHandle
-getDbHandle = go =<< Annex.getState Annex.keysdbhandle
-  where
-	go (Just h) = pure h
-	go Nothing = do
-		h <- liftIO newDbHandle
-		Annex.changeState $ \s -> s { Annex.keysdbhandle = Just h }
-		return h
-
 {- Opens the database, perhaps creating it if it doesn't exist yet.
  -
  - Multiple readers and writers can have the database open at the same
@@ -153,9 +142,7 @@ openDb createdb _ = catchPermissionDenied permerr $ withExclusiveLock gitAnnexKe
  - data to it.
  -}
 closeDb :: Annex ()
-closeDb = Annex.getState Annex.keysdbhandle >>= \case
-	Nothing -> return ()
-	Just h -> liftIO (closeDbHandle h)
+closeDb = liftIO . closeDbHandle =<< Annex.getState Annex.keysdbhandle
 
 addAssociatedFile :: Key -> TopFilePath -> Annex ()
 addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile k f