[region-isolation] Shrink PartitionUtils.h by moving large functions to cpp file.

Just making PartitionUtils.h a little easier to walk through by moving more of
the impl into the .cpp file. This reduces the header from ~1500 lines to ~950
lines which is more managable. This is especially important since I am going
to be adding IsolationHistory to the header file which will expand it even
further.

(cherry picked from commit 901c7c2)

Author: gottesmm