Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.
This is a function that definitely doesn't always have a non-modifying
behavior across all implementations, so this should be made non-const.
This gets rid of the need to mark data members as mutable to work around
the fact mutating data members needs to occur.
Zach Hilman