ACCUMULO-2985 Ensure that ES is properly initialized during start()

By initializing the ExecutorService in the constructor, I broke the ability
to stop, start, and then re-stop the minicluster. Lazily initialize the ES
in start() which should alleviate the issue.
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT

Conflicts:
	minicluster/src/main/java/org/apache/accumulo/minicluster/MiniAccumuloCluster.java
	minicluster/src/test/java/org/apache/accumulo/minicluster/impl/CleanShutdownMacTest.java
Merge branch '1.6.1-SNAPSHOT'