Revert "Merge pull request #495 from metamx/dim-extraction-null-values"

This reverts commit 76437379d7eee80898bd70f589b0eefd393e9f90, reversing
changes made to 79e6d4eb56806e9ab123a93458c99efd1ea2af03.

Conflicts:
	pom.xml
revert changes to hyperUnique
[maven-release-plugin] prepare release druid-0.6.108
[maven-release-plugin] prepare for next development iteration
initial support for LZ4 compression