ACCUMULO-1817 Introduce usage of Hadoop Metrics2 by default for JMX and other metrics sinks (graphite and ganglia)

Preserves "old" custom JMX MBean registration via Property.general.legacy.metrics, otherwise, Metrics2
classes are used. Example hadoop-metrics2-accumulo.properties configuration file is provided, which
enables nothing but serves as a default for users to enable file collection, graphite, or ganglia.

Cleaned up a bit of ill-formatted code, consolidated utility methods and constants across both implementations,
and introduced factory methods to create each type of metric collection class we provide.