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.

Remote/P2P.hs

@@ -35,14 +35,14 @@ remote = RemoteType
 	-- Remote.Git takes care of enumerating P2P remotes,
 	-- and will call chainGen on them.
 	, enumerate = const (return [])
-	, generate = \_ _ _ _ -> return Nothing
+	, generate = \_ _ _ _ _ -> return Nothing
 	, setup = error "P2P remotes are set up using git-annex p2p"
 	, exportSupported = exportUnsupported
 	, importSupported = importUnsupported
 	}
 
-chainGen :: P2PAddress -> Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remote)
-chainGen addr r u c gc = do
+chainGen :: P2PAddress -> Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> RemoteStateHandle -> Annex (Maybe Remote)
+chainGen addr r u c gc rs = do
 	connpool <- mkConnectionPool
 	cst <- remoteCost gc veryExpensiveRemoteCost
 	let protorunner = runProto u addr connpool
@@ -76,6 +76,7 @@ chainGen addr r u c gc = do
 		, getInfo = gitRepoInfo this
 		, claimUrl = Nothing
 		, checkUrl = Nothing
+		, remoteStateHandle = rs
 	}
 	return (Just this)