For ENG-4829, replace COWMap/Set with Cliff Click's NonBlockingHashMap/Set.
Fix fired asserts. Asserts get fired because EstTime is no longer
used for m_lastPendingWriteTime, so we can not predict accurate
interval number now.
Revert back to EstTime in NIOWriteStream.
Replace ConcurrentSkipListMap with NonBlockingHashMap, and add third party license.
Merge branch 'master' into ENG-4829

Conflicts:
	src/frontend/org/voltdb/AdmissionControlGroup.java
Merge pull request #595 from VoltDB/ENG-4829

ENG-4829: Replace COWMap/Set with Cliff Click NonBlockingHashMap/Set