Support opportunistic locks in mcpu/LUs

Similar to “share_locks”, a new dictionary containing booleans for each
locking level is added to “cmdlib.LogicalUnit”. Logical units wanting to
make use of opportunistic locks will be able to configure this
dictionary accordingly.

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