ACCUMULO-2870 Fail hard and fast if we don't calculate a midpoint.

By failing early, we catch the change in functionality from FileUtil.findMidPoint
which always returns a mid-point. Also prevents an NPE from unexpectedly being thrown.
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT

Conflicts:
	server/tserver/src/main/java/org/apache/accumulo/tserver/Tablet.java
Merge branch '1.6.1-SNAPSHOT'
ACCUMULO-2919 Remove references to logger in JMX configs and documentation
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT

Conflicts:
	conf/examples/1GB/standalone/accumulo-metrics.xml
	conf/examples/2GB/native-standalone/accumulo-metrics.xml
	conf/examples/2GB/standalone/accumulo-metrics.xml
	conf/examples/3GB/native-standalone/accumulo-metrics.xml
	conf/examples/3GB/standalone/accumulo-metrics.xml
	conf/examples/512MB/native-standalone/accumulo-metrics.xml
	conf/examples/512MB/standalone/accumulo-metrics.xml
Merge branch '1.6.1-SNAPSHOT'