ACCUMULO-1798 Add ability to specify compaction strategy for user specificed compactions.
ACCUMULO-3385 Fix broken DateLexicoder ordering

  Corrects the ordering so that dates before 1970 actually sort before dates
  after it. This changes the internal serialization, so please see the release
  notes for a workaround if you have data written with the buggy form.
Merge branch '1.6'

Conflicts:
	core/src/test/java/org/apache/accumulo/core/client/lexicoder/ReverseLexicoderTest.java
ACCUMULO-1798 Remove unused import
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.