refactor ref change watching

Added to change notification to P2P protocol.

Switched to a TBChan so that a single long-running thread can be
started, and serve perhaps intermittent requests for change
notifications, without buffering all changes in memory.

The P2P runner currently starts up a new thread each times it waits
for a change, but that should allow later reusing a thread. Although
each connection from a peer will still need a new watcher thread to run.

The dependency on stm-chans is more or less free; some stuff in yesod
uses it, so it was already indirectly pulled in when building with the
webapp.

This commit was sponsored by Francois Marier on Patreon.

RemoteDaemon/Types.hs

@@ -5,7 +5,6 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 
 module RemoteDaemon.Types where
@@ -15,6 +14,7 @@ import qualified Annex
 import qualified Git.Types as Git
 import qualified Utility.SimpleProtocol as Proto
 import Types.GitConfig
+import Annex.ChangedRefs (ChangedRefs)
 
 import Network.URI
 import Control.Concurrent
@@ -52,13 +52,11 @@ data Consumed
 	= PAUSE
 	| LOSTNET
 	| RESUME
-	| CHANGED RefList
+	| CHANGED ChangedRefs
 	| RELOAD
 	| STOP
 	deriving (Show)
 
-type RefList = [Git.Ref]
-
 instance Proto.Sendable Emitted where
 	formatMessage (CONNECTED remote) =
 		["CONNECTED", Proto.serialize remote]
@@ -100,10 +98,6 @@ instance Proto.Serializable RemoteURI where
 	serialize (RemoteURI u) = show u
 	deserialize = RemoteURI <$$> parseURI
 
-instance Proto.Serializable RefList where
-	serialize = unwords . map Git.fromRef
-	deserialize = Just . map Git.Ref . words
-
 instance Proto.Serializable Bool where
 	serialize False = "0"
 	serialize True = "1"