Added support for a special JDK8+ build which will use LongAdder.

LongAdder has better performance characteristics under contention than
AtomicLong.
Maven build has been enhanced to support two profiles: java-6 and java-8
These profiles include a different set of source paths. java-6 is the
default and the artefact will look identical to ones build prior this
commit.
To avoid duplication a TrackerConfig class has been introduced to hold
constants.
Because the Tracker is statically used, there is no easy way to reuse
functionality by inheritance. Any refactorings to enable that would
sacrifice performance. The amount of duplication is however quite limited.