JENA-813 : Use IteratorConcat for iteraing over all graphs.
JENA-813 : Add comments on usage.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/jena
Improve performance of DatasetGraphCollection (JENA-813)

Switches it to use IteratorConcat instead of IteratorCons.  Also
deprecates and switches all usages of IteratorCons to IteratorConcat.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/jena

Merge together mine and Andy's javadoc changes

Conflicts:
	jena-arq/src/main/java/com/hp/hpl/jena/sparql/core/DatasetGraphCollection.java
	jena-arq/src/main/java/org/apache/jena/atlas/iterator/Iter.java
	jena-arq/src/main/java/org/apache/jena/atlas/iterator/IteratorConcat.java