Allow locking to be used via OpQuery

The original design for query2 specifically excluded locking, but now
it's turned out that it would be a good thing to have in watcher. This
patch adds a new parameter to OpQuery and enables its use in LUQuery. A
missing function is added to LUGroupQuery, a comment clarified in
_NodeQuery and all locks declared as shared acquires in the same LU.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>