Remove thread pools from MetricsRegistry.

Meters no longer require tick threads, but instead perform the exponential decay on reads and marks.

This allows us to remove all thread pools from the MetricsRegistry level and push it down into AbstractPollingReporter.

This also means we can remove MetricsServletContextListener, since there are no more lifecycle issues.

Conflicts:
	metrics-core/src/main/java/com/yammer/metrics/Metrics.java
	metrics-core/src/main/java/com/yammer/metrics/core/Meter.java
	metrics-core/src/main/java/com/yammer/metrics/core/Timer.java
	metrics-core/src/test/java/com/yammer/metrics/core/tests/TimerTest.java
	metrics-servlet/src/main/java/com/yammer/metrics/reporting/MetricsServletContextListener.java
	metrics-servlet/src/test/java/com/yammer/metrics/reporting/tests/MetricsServletContextListenerTest.java
[maven-release-plugin] prepare release v2.2.1
[maven-release-plugin] rollback the release of v2.2.1
sumofy
[maven-release-plugin] prepare release v2.2.1.sumo