Replace EstTime with System time in NIOWriteStream; replace COWMap with ConcurrentHashMap.
Replace inner map with concurrent map in AdmissionControlGroup.
Rename variables in DummyIterator of InitiatorStats.
fix wrong order when inserting to m_connectionStats
Remove unnecessary imports and fix junit test for NIOWriteStream.
Revert back to EstTime in NIOWriteStream.
Merge pull request #585 from VoltDB/concurrentHashMap

ENG-4804: Replace COWMap with java ConcurrentHashMap