ACCUMULO-3306 Simplify PortUtils.getRandomFreePort

Remove the computation of a "random" port number
and just use the OS provided feature of asking
to bind the socket on '0'. Fixed a bug where
the count variable was not incremented.
ACCUMULO-3308 Expand docs on configuration of IMM.

Expand on how the minc threshold, IMM size and WAL size
must be altered together. Additionally, mention how
the HDFS block size for the WAL should be increased to 2GB
when the WAL size is increased over 2GB.
Merge branch '1.6'

Conflicts:
	docs/src/main/latex/accumulo_user_manual/chapters/administration.tex
ACCUMULO-3308 Clarify WAL block size recommendation.
Merge branch '1.6'

Conflicts:
	docs/src/main/latex/accumulo_user_manual/chapters/administration.tex