Add the `volatile` qualifier to shared memory pointers

[maxwellpirtle]

After reading a bit more about the volatile keyword in a couple of places, it seems that we need to mark any data we have in shared memory as volatile to prevent the compiler from making an optimization it shouldn't make. We'd effectively be marking our data as being allowed to be changed outside of the current program or in ways that the compiler can't determine statically, if I understand correctly. This should be a pretty small addition.

[gc00]

@maxwellpirtle , Yes, we need to use volatile wherever some date is shared among more than one thread.

Actually, there are rules that often do the right thing for us. For example, if there is a new function call (or finishing a function), then all registers are again written to memory. So, the issue usually arises when a reference to shared data occurs and we are worried about a race condition while the thread is still within the same function. This is not a common situation, but it is important to recognize it when it happens. And the performance penalty of declaring a variable as volatile is small.