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.

Command/P2PStdIO.hs

@@ -11,13 +11,10 @@ import Command
 import P2P.IO
 import P2P.Annex
 import qualified P2P.Protocol as P2P
-import Git.Types
 import qualified Annex
 import Annex.UUID
 import qualified CmdLine.GitAnnexShell.Checks as Checks
 import qualified CmdLine.GitAnnexShell.Fields as Fields
-import Utility.AuthToken
-import Utility.Tmp.Dir
 
 cmd :: Command
 cmd = noMessages $ command "p2pstdio" SectionPlumbing
@@ -38,7 +35,9 @@ start = do
 		Just u -> return (toUUID u)
 	myuuid <- getUUID
 	conn <- stdioP2PConnection <$> Annex.gitRepo
-	let server = P2P.serveAuthed servermode myuuid
+	let server = do
+		P2P.net $ P2P.sendMessage (P2P.AUTH_SUCCESS myuuid)
+		P2P.serveAuthed servermode myuuid
 	runFullProto (Serving theiruuid Nothing) conn server >>= \case
 		Right () -> next $ next $ return True
 		Left e -> giveup e