p2p ssh connection pools

Much like Remote.P2P, there's a pool of connections to a peer, in order
to support concurrent operations.

Deals with old git-annex-ssh on the remote that does not support p2pstdio,
by only trying once to use it, and remembering if it's not supported.

Made p2pstdio send an AUTH_SUCCESS with its uuid, which serves the dual
purposes of something to detect to see that the connection is working,
and a way to verify that it's connected to the right uuid.
(There's a redundant uuid check since the uuid field is sent
by git_annex_shell, but I anticipate that being removed later when
the legacy git-annex-shell stuff gets removed.)

Not entirely happy with Remote.Git.runSsh's behavior
when the proto action fails. Running the fallback will work ok, but what
will we do when the fallbacks later get removed? It might be better to
try to reconnect, in case the connection got closed.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

P2P/IO.hs

@@ -10,6 +10,7 @@
 module P2P.IO
 	( RunProto
 	, P2PConnection(..)
+	, ClosableConnection(..)
 	, stdioP2PConnection
 	, connectPeer
 	, closeConnection
@@ -51,6 +52,10 @@ data P2PConnection = P2PConnection
 	, connOhdl :: Handle
 	}
 
+data ClosableConnection conn
+	= OpenConnection conn
+	| ClosedConnection
+
 -- P2PConnection using stdio.
 stdioP2PConnection :: Git.Repo -> P2PConnection
 stdioP2PConnection g = P2PConnection