Improve compaction

[ambud]

Compaction right now is a bit sensitive to running large number of buffer re-writes. Compaction's goal is to reduce the data size and reduce writer list sizes that saves heap and allows deletion of older files.

Enhancement would allow Sidewinder to reduce heap utilization while preventing dangling old data files that still require memory mapping.

[pauljv92]

while autocorrelation w GPU takes it own sweet time, Im down to look at this! :) pls help to point me to the code I should be looking at