it forces the user to bzero a key before setting it as a pointer, otherwise
it'll have random content on architectures where
sizeof(void*) < sizeof(ulonglong).

Declaring a key as ulonglong only (not a union) makes this user mistake
impossible.

include/waiting_threads.h:
  WT_RESOURCE_ID::value is an ulonglong, not a union
mysys/waiting_threads.c:
  WT_RESOURCE_ID::value is an ulonglong, not a union
storage/maria/ma_write.c:
  WT_RESOURCE_ID::value is an ulonglong, not a union
storage/maria/trnman.c:
  WT_RESOURCE_ID::value is an ulonglong, not a union
unittest/mysys/waiting_threads-t.c:
  WT_RESOURCE_ID::value is an ulonglong, not a union