[FLINK-12987][metrics] fix DescriptiveStatisticsHistogram#getCount() not returning the number of elements seen

DescriptiveStatisticsHistogram#getCount() only returned the number of currently
stored elements, not the total number seen.

Also add a unit test for verifying any Histogram implementation (based on
the previous test from DropwizardFlinkHistogramWrapperTest) and run this with
DescriptiveStatisticsHistogram.

This moves the API compatibility checks into the API projects that use stability annotations.
Previously, every project ran the tests, regardless of whether it contained public API classes or not.

This closes #2334

This closes #2324

- moved user-facing API to 'flink-metrics/flink-metrics-core'
- moved JMXReporter to 'flink-metrics/flink-metrics-jmx'
- moved remaining metric classes to 'flink-runtime'

This closes #2226

This closes #2112

  - Metric groups are generally thread-safe
  - Metric groups are closable. Closed groups do not register metric objects and more.
  - TaskManager's JobMetricsGroup auto disposes when all TaskMetricGroups are closed
  - Maven project with metric reporters renamed to 'flink-metric-reporters'
  - Various code style cleanups

This closes #1427

0.10-SNAPSHOT continues on branch release-0.10

Closes #851

Default hadoop version is now hadoop2
Generate specific pom avoids version collisions with dependencies
Updated poms for new release scripts

Conflicts:
	flink-addons/flink-avro/pom.xml
	flink-addons/flink-hadoop-compatibility/pom.xml
	flink-addons/flink-hbase/pom.xml
	flink-addons/flink-jdbc/pom.xml
	flink-addons/flink-spargel/pom.xml
	flink-addons/flink-tachyon/pom.xml
	flink-quickstart/flink-quickstart-java/pom.xml
	flink-quickstart/flink-quickstart-scala/pom.xml

Removed Disclaimer file
Eliminated unnecessary incubating substrings
Bumped version to 0.9-SNAPSHOT

Replacing the EventCollector and the MemoryArchivist.

Finished EventCollector and adjusted ExecutionGraph and ExecutionVertex to register actors as listeners.

- update _config.yaml of integrated documentation
- quick check on the quickstarts / build instructions to match the latest release
- quickstart.sh scrips now point to the correct versions.

source should now build without the presence of a .git directory
included the quickstart-archetypes into the main project.