Motivation:
in a highly concurrent environment the ConcurrentHashMap hash map has
a better throughput than synchronized hashmap.

Benchmark                                                (map)   Mode  Cnt       Score       Error  Units
FileTrackerBenchmark.fileTrackerHashMapTest      HashMap  thrpt   25  691051.876 ± 23720.699  ops/s
FileTrackerBenchmark.fileTrackerHashMapTest  ConcHashMap  thrpt   25  792051.898 ± 18111.740  ops/s

Benchmark with 48 concurrent clients.

Modification:
Update FileTracker to use ConcurrentHashMap to track open files

Result:
better throughput in concurrent environment

Acked-by: Lea Morschel
Target: master