ISPN-4640 Enhance distributed iterator to allow filter and converter to

be executed in the same step/invocation

* Added new KeyValueFilterConverter interface
* Both Local and Distributed Iterators support new interface

https://issues.jboss.org/browse/ISPN-4640
I believe we should also say that the input value is never null, here and in KeyValueFilter.accept, Converter.convert,
and also Converter.convert should never return null.