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