Ssh password prompting improved when using -J

When ssh connection caching is enabled (and when GIT_ANNEX_USE_GIT_SSH is
not set), only one ssh password prompt will be made per host, and only one
ssh password prompt will be made at a time.

This also fixes a race in prepSocket's stale ssh connection stopping
when run with -J. It was possible for one thread to start a cached ssh
connection, and another thread to immediately stop it, resulting in excess
connections being made.

This commit was supported by the NSF-funded DataLad project.

Annex/LockFile.hs

@@ -11,6 +11,7 @@ module Annex.LockFile (
 	lockFileCached,
 	unlockFile,
 	getLockCache,
+	fromLockCache,
 	withExclusiveLock,
 	tryExclusiveLock,
 ) where