We can now relax the one-region-one-task rule.

Small clusters will find it useful to give a task multiple regions, and the scheduling code makes a reasonable effort to ensure all of those regions are task-local.
Hoist ScanSpout::validateConfig to HBaseSpout.
doco
Hoist ScanSpout::configureTupleMapper to be default impl.
Add a TupleMapper::map overload that ignors the input tuple.