OK, I made this change, and they're back to being macro calls.

What I did was push the "are statistics configured" test down into the underlying WT_STAT_XXX macros, which means we will test if statistics are configured every single time we update something, but  this ensures we won't actually write the shared memory unless stats are, in fact, configured.   Hopefully the compiler will optimize out the extra tests and we'll only test if stats are configured once per function.