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
ACCUMULO-3215 Ensure that ImportTable writes absolute paths, not relative.

ImportExportIT creates a table, exports it and then imports it to the
same instance to ensure that the imported copy is equivalent
to the original. Without the changes to ImportTable, the test failed
because the copy was unusable due to incorrect metadata table entries.
Merge branch '1.6'

Conflicts:
	server/master/src/main/java/org/apache/accumulo/master/tableOps/ImportTable.java
ACCUMULO-3217 Fix use of deprecated members
Merge branch '1.6'