WithLock is for use solely to declare a dummy argument to functions that must be called while
holding a lock, as a rigorous alternative to an unchecked naming convention and/or comments to
indicate the requirement.

It may be used to modernize code from (something like) this

    // Member _mutex must be held when calling this.
    void _init_inlock(OperationContext* opCtx) {
        _stuff = makeStuff(opCtx);
    }

to

    void _init(WithLock, OperationContext* opCtx) {
        _stuff = makeStuff(opCtx);
    }

The call to such a function looks like this:

    stdx::lock_guard<stdx::mutex> lk(_mutex);
    _init(lk, opCtx);  // instead of _init_inlock(opCtx)

Note that the formal argument need not (and should not) be named unless it is needed to pass
along to another function:

    void _init(WithLock lock, OperationContext* opCtx) {
        _really_init(lock, opCtx);
    }

~
~