create set of fixed slots based off of numprocesses

allow reuse of slots as Process objects are reaped
Revert "create set of fixed slots based off of numprocesses"

This reverts commit aea929e7e6ca679b09d8fe4e0e2af08a747bc6d5.
supply a worker id that tracks w/ numprocesses

worker id's are reused as their corresponding process is reaped
a new property is added: "Watcher.nextwid" which returns the lowest unused worker id number
name change for nextwid

implied private property
removed unused var
ensure wid re-use

- check max worker id
- calulate checksum of worker ids and compare against expected range of
  worker ids
Merge pull request #513 from davidbirdsong/birdsong-wid-slots

create set of fixed slots based off of numprocesses