ACCUMULO-2887 Add ObservableConfiguration

The logic for maintaining and notifying observers in NamespaceConfiguration and
TableConfiguration is refactored into a new ObservableConfiguration class.
Merge branch '1.6.1-SNAPSHOT'

Conflicts:
	server/base/src/main/java/org/apache/accumulo/server/conf/NamespaceConfiguration.java
	server/base/src/main/java/org/apache/accumulo/server/conf/TableConfiguration.java