add support for chaining Maps in a single Map and after a Reduce [M*/RM*]

On the same input, we usually need to run multiple Maps one after the other without no Reduce. We also have to run multiple Maps after the Reduce.

If all pre-Reduce Maps are chained together and run as a single Map a significant amount of Disk I/O will be avoided.

Similarly all post-Reduce Maps can be chained together and run in the Reduce phase after the Reduce.