Misc improvements to PositiveIntSet

In support of UIMA-3399 and UIMA-4059, improve the PositiveIntSet switching between underlying formats: have the changed-to format have some extra room so that it doesn't immediately need to expand. Recognize that the underlying formats typically do "doubling" of the space used, and add a fast-path if an add operation won't trigger this.

Enhance the interface so it can be used for the FSBagIndex underlying impl, replacing IntVector, in order to support UIMA-3399.