Reduce variance by (1) Using common chunks at the end (2) Busy looping (#486)

on automatic closing of ByteBuffers.. previously, a straggler could hold
up closing the ByteBuffers.

Also
- Improve Tracing code
- Parametrize additional options to aid in tuning

Our previous PR was surprising; parallelizing munmap() call did not
yield anywhere near the performance gain I expected. Local machine had
10% gain while testing machine only showed 2% gain. I am still not clear
why it happened and the two best theories I have are
1) Variance due to stragglers (that this change addresses)
2) munmap() is either too fast or too slow relative to the other
   instructions compared to our local machine. I don't know which. We'll
   have to use adaptive tuning, but that's in a different change.

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