ACCUMULO-3217 Make KeyValue extend SimpleImmutableEntry instead of implement Map.Entry.

We get toString, hashCode and equals for free by extending SimpleImmutableEntry
over implementing Map.Entry. Without this change, users couldn't properly
use the Entry<Key,Value> objects that were returned by Scanners and BatchScanners.
Merge branch '1.5' into 1.6

Conflicts:
	server/tserver/src/main/java/org/apache/accumulo/tserver/Tablet.java
Merge branch '1.6'

Conflicts:
	core/src/main/java/org/apache/accumulo/core/data/Key.java
	core/src/main/java/org/apache/accumulo/core/data/KeyValue.java
	server/tserver/src/main/java/org/apache/accumulo/tserver/TabletServer.java
	server/tserver/src/main/java/org/apache/accumulo/tserver/tablet/Tablet.java