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/List.hs

@@ -1,12 +1,12 @@
-{-# LANGUAGE CPP #-}
-
 {- git-annex remote list
  -
- - Copyright 2011,2012 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2019 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Remote.List where
 
 import qualified Data.Map as M
@@ -15,6 +15,7 @@ import Annex.Common
 import qualified Annex
 import Logs.Remote
 import Types.Remote
+import Types.RemoteState
 import Annex.UUID
 import Remote.Helper.Hooks
 import Remote.Helper.ReadOnly
@@ -106,10 +107,11 @@ remoteGen m t g = do
 	u <- getRepoUUID g
 	gc <- Annex.getRemoteGitConfig g
 	let cu = fromMaybe u $ remoteAnnexConfigUUID gc
+	let rs = RemoteStateHandle cu
 	let c = fromMaybe M.empty $ M.lookup cu m
-	generate t g u c gc >>= \case
+	generate t g u c gc rs >>= \case
 		Nothing -> return Nothing
-		Just r -> Just <$> adjustExportImport (adjustReadOnly (addHooks r))
+		Just r -> Just <$> adjustExportImport (adjustReadOnly (addHooks r)) rs
 
 {- Updates a local git Remote, re-reading its git config. -}
 updateRemote :: Remote -> Annex (Maybe Remote)