Add simple lock monitor

This patch adds an initial implementation of a lock monitor, accessible
for the user through “gnt-debug locks”. It currently shows all resource
locks: BGL, nodes and instances. Config and job queue locks could be
shown too, but wouldn't be of much help.  The current owner(s) and mode
are also shown.

Showing pending acquires will require further changes on the SharedLock
internals and is not yet implemented.

Example output:
$ gnt-debug locks -o name,mode,owner
Name            Mode      Owner
BGL/BGL         shared    JobQueue19/Job147
instances/inst1 exclusive JobQueue19/Job147
instances/inst2 -         -
instances/inst3 -         -
instances/inst4 -         -
nodes/node1     exclusive JobQueue19/Job147
nodes/node2     exclusive JobQueue19/Job147

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>