Polished SMP caching code, primarily to stay out of the way in non-SMP mode.

Polished SMP caching code, primarily to stay out of the way in non-SMP mode.

Do not start useless diskers. Do not assume Rock cache_dirs are present.
Do not require IpcIo DiskIO module to build Rock store.
Check IPC I/O pages limits in Rock store only when using a disker.
Warn about Rock cache_dir disk space waste.
Warn if shared memory cache is enabled in non-SMP mode.
Fake shared memory segments if needed (e.g., we are using Rock cache_dirs with
  no POSIX shared memory support) and possible (e.g., no SMP).