locking.LockSet: don't modify input arguments

locking.LockSet: don't modify input arguments

Currently LockSet.acquire() sorts in place it's input argument if it's a
list. This is not good, since callers might depend on a specific
ordering of the input data, and this is a 'hidden' modification.

We fix it by simply using a sorted copy, instead of sorting in place.

Reviewed-by: ultrotter