roxiemem - improve bitmap processing code

- Clean up various items highlighted by previous code reviews
- Change activityIds to allocatorIds to make distinction clearer
- Use get_cycles_now instead of time, and avoid calling if not
  needed.
- Move the critical section in the bitmap code to reduce scope

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>