locking: Allow checking if lock is owned in certain mode

With this patch the “LockSet” and “GanetiLockManager” classes have a new
function to check if a single or a group of locks (at a certain level)
have been acquired in a specific mode. This will be used for additional
assertions. Until now they could only check if a lock has been acquired,
but not in which mode. One use-case will be updating the node state in
various places, where the node lock must be acquired in exclusive mode.

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