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.

P2P/Annex.hs

@@ -16,6 +16,7 @@ module P2P.Annex
 import Annex.Common
 import Annex.Content
 import Annex.Transfer
+import Annex.ChangedRefs
 import P2P.Protocol
 import P2P.IO
 import Logs.Location
@@ -114,6 +115,14 @@ runLocal runmode runner a = case a of
 				protoaction False
 				next
 			Right _ -> runner next
+	WaitRefChange next -> do
+		v <- tryNonAsync $ bracket
+			watchChangedRefs
+			(liftIO . stopWatchingChangedRefs)
+			(liftIO . waitChangedRefs)
+		case v of
+			Left e -> return (Left (show e))
+			Right changedrefs -> runner (next changedrefs)
   where
 	transfer mk k af ta = case runmode of
 		-- Update transfer logs when serving.