ACCUMULO-2825 Add RowEncodingIterator

The WholeRowIterator now extends the abstract RowEncodingIterator
Update iterator javadoc
ACCUMULO-2825 Fix formatting and remove javadoc warnings
Merge branch 'ACCUMULO-2825' into 1.6.1-SNAPSHOT

Signed-off-by: Josh Elser <elserj@apache.org>