ACCUMULO-1921 check for null
ACCUMULO-1937 decrease log severity for expected case of a tablet no longer being in the unopened set at the moment it is being loaded

Conflicts:
	server/src/main/java/org/apache/accumulo/server/tabletserver/TabletServer.java
ACCUMULO-2183 Remove the .equals in favor of an equality check that enforces compile-time type checking.