Changed Comparators for Ordering, and moved the class into the util
package with SortedSet and SortedCollection.
Changed Comparable/Comparator implementations for 'Ordering'
implementation which does not have the same contract as Comparable.
Added SortedCollectionConcurrencyTest, to track down possible issue with
invalid insertion order under concurrency; synchronized the 'add'
methods on SortedCollection and SortedSet to fix bugs shown by this
test.
Cleaned up imports