use lock pools throughout git-annex

The one exception is in Utility.Daemon. As long as a process only
daemonizes once, which seems reasonable, and as long as it avoids calling
checkDaemon once it's already running as a daemon, the fcntl locking
gotchas won't be a problem there.

Annex.LockFile has it's own separate lock pool layer, which has been
renamed to LockCache. This is a persistent cache of locks that persist
until closed.

This is not quite done; lockContent stil needs to be converted.

Utility/LockFile.hs

@@ -4,6 +4,9 @@
  - This module does *not* attempt to be a portability shim, it just exposes
  - the native locking of the OS.
  -
+ - Posix fcntl locks have some gotchas. So, consider using
+ - Utility.LockPool instead of using this module directly.
+ -
  - Copyright 2014 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause