opStrategy and ordering for ArithmeticPostAgg

Fixes #510
reduce object count overhead of column cache
add approximate object overhead
formatting
disable column cache by default
add column cache docs
fix NPE while indexing
add more logging for groupby caching
Merge pull request #675 from metamx/slimmer-column-cache

reduce object count overhead of column cache
prepare for next release
[maven-release-plugin] prepare release druid-0.6.144
[maven-release-plugin] prepare for next development iteration
Merge branch 'master' into druid-0.7.x

Conflicts:
	cassandra-storage/pom.xml
	common/pom.xml
	examples/config/historical/runtime.properties
	examples/config/overlord/runtime.properties
	examples/config/realtime/runtime.properties
	examples/pom.xml
	hdfs-storage/pom.xml
	histogram/pom.xml
	indexing-hadoop/pom.xml
	indexing-service/pom.xml
	kafka-eight/pom.xml
	kafka-seven/pom.xml
	pom.xml
	processing/pom.xml
	rabbitmq/pom.xml
	s3-extensions/pom.xml
	server/pom.xml
	services/pom.xml
fix compilation with Java 8
some better null handling for MV
fix all manners of brokenness from nulls and empty strings
fix compilation with Java 8
Merge branch 'druid-0.7.x' into offheap-incremental-index
fix some rt cleanup and failure edge cases
address cr