Ensure that all direct uses of spinlock-protected data structures use

Ensure that all direct uses of spinlock-protected data structures use
'volatile' pointers to access those structures, so that optimizing
compilers will not decide to move the structure accesses outside of the
spinlock-acquire-to-spinlock-release sequence.  There are no known bugs
in these uses at present, but based on bad experience with lwlock.c,
it seems prudent to ensure that we protect these other uses too.
Per pghackers discussion around 12-Dec.  (Note: it should not be
necessary to worry about structures protected by LWLocks, since the
LWLock acquire and release operations are not inline macros.)