external special remotes mostly implemented (untested)

This has not been tested at all. It compiles!

The only known missing things are support for encryption, and for get/set
of special remote configuration, and of key state. (The latter needs
separate work to add a new per-key log file to store that state.)

Only thing I don't much like is that initremote needs to be passed both
type=external and externaltype=foo. It would be better to have just
type=foo

Most of this is quite straightforward code, that largely wrote itself given
the types. The only tricky parts were:

* Need to lock the remote when using it to eg make a request, because
  in theory git-annex could have multiple threads that each try to use
  a remote at the same time. I don't think that git-annex ever does
  that currently, but better safe than sorry.

* Rather than starting up every external special remote program when
  git-annex starts, they are started only on demand, when first used.
  This will avoid slowdown, especially when running fast git-annex query
  commands. Once started, they keep running until git-annex stops, currently,
  which may not be ideal, but it's hard to know a better time to stop them.

* Bit of a chicken and egg problem with caching the cost of the remote,
  because setting annex-cost in the git config needs the remote to already
  be set up. Managed to finesse that.

This commit was sponsored by Lukas Anzinger.

Assistant/WebApp/RepoList.hs

@@ -223,7 +223,7 @@ getRepositoriesReorderR = do
 		rs <- catMaybes <$> mapM Remote.remoteFromUUID list
 		forM_ (reorderCosts remote rs) $ \(r, newcost) ->
 			when (Remote.cost r /= newcost) $
-				setRemoteCost r newcost
+				setRemoteCost (Remote.repo r) newcost
 		void remoteListRefresh
   	fromjs = toUUID . T.unpack