Package com.ibm.jbatch.container.callback

This package has not been reviewed for correctness. This was part of the RI's implementation of the TCK SPI. Then we switched to polling from the TCK without requiring any RI-specific code. Still, polling, while nice from the TCK perspective, has its disadvantages for a real runtime where some other mechanism might be preferred. Since we spent some time getting synchronization correct we decided to hold onto this code and keep it in the RI, possibly for future use.