add RemoteStateHandle

This solves the problem of sameas remotes trampling over per-remote
state. Used for:

* per-remote state, of course
* per-remote metadata, also of course
* per-remote content identifiers, because two remote implementations
  could in theory generate the same content identifier for two different
  peices of content

While chunk logs are per-remote data, they don't use this, because the
number and size of chunks stored is a common property across sameas
remotes.

External special remote had a complication, where it was theoretically
possible for a remote to send SETSTATE or GETSTATE during INITREMOTE or
EXPORTSUPPORTED. Since the uuid of the remote is typically generate in
Remote.setup, it would only be possible to pass a Maybe
RemoteStateHandle into it, and it would otherwise have to construct its
own. Rather than go that route, I decided to send an ERROR in this case.
It seems unlikely that any existing external special remote will be
affected. They would have to make up a git-annex key, and set state for
some reason during INITREMOTE. I can imagine such a hack, but it doesn't
seem worth complicating the code in such an ugly way to support it.

Unfortunately, both TestRemote and Annex.Import needed the Remote
to have a new field added that holds its RemoteStateHandle.

Annex/Import.hs

@@ -382,7 +382,7 @@ downloadImport remote importtreeconfig importablecontents = do
 			getTopFilePath subdir </> fromImportLocation loc
 
 	getcidkey cidmap db cid = liftIO $
-		CIDDb.getContentIdentifierKeys db (Remote.uuid remote) cid >>= \case
+		CIDDb.getContentIdentifierKeys db rs cid >>= \case
 			[] -> atomically $
 				maybeToList . M.lookup cid <$> readTVar cidmap
 			l -> return l
@@ -390,8 +390,10 @@ downloadImport remote importtreeconfig importablecontents = do
 	recordcidkey cidmap db cid k = do
 		liftIO $ atomically $ modifyTVar' cidmap $
 			M.insert cid k
-		liftIO $ CIDDb.recordContentIdentifier db (Remote.uuid remote) cid k
-		CIDLog.recordContentIdentifier (Remote.uuid remote) cid k
+		liftIO $ CIDDb.recordContentIdentifier db rs cid k
+		CIDLog.recordContentIdentifier rs cid k
+
+	rs = Remote.remoteStateHandle remote
 
 {- Temporary key used for import of a ContentIdentifier while downloading
  - content, before generating its real key. -}