improvements to DeterministicSchedule

Summary: This diff isolates the CacheLocality back end for DeterministicSchedule
from one invocation to the next (the last one was deterministic
across the entire program but not per-DeterministicSchedule instance),
and makes it easy to do extensive tracing of memory accesses during
a deterministic test.  These changes were made while tracking down a
bug, but don't fix any bugs on their own (and in fact don't affect any
production code at all).

Reviewed By: @yfeldblum

Differential Revision: D1842390