Introduce AirConcurrentMap as possible backing HashMap

[pjeli]

Ever since #230 and #293, I have been interested in trying to find optimizations to the backing HashMap that separates files and directories of the LightweightGSet.

I have always allowed folks to try their own hand at providing a faster implementation by the configuration property nna.inode.collection.impl.

Well at long last I think I finally have one. The early results look promising: https://github.com/boilerbay/airconcurrentmap

From the ValidQueryChecker benchmark and the GSetFilteringBenchmark underneath src/test/java I have seen an 18->13 second reduction for ValidQuery and a 36->30 second reduction for GSetFiltering. These speed-ups have been consistent as I tested with 1M files, 2M files, and 4M files.

I think the results are good enough that I will soon try a cut of this on a much larger system.

[pjeli]

Unfortunately this did not end up having the performance increase I was expecting (20 second SuggestionEngine runs became 2 minutes). I think it may be due to the way Streams and AirConcurrentMap interact. Unfortunately I can't make much more progress since Air is closed source. So debugging is a pain. I will leave this alone for now since it does work though. If anyone wants to take a crack at it go ahead.