add before/after batch hooks
avoid possible hanging if some workers crash
syntax error?
add batch to terrain (not very DRY, is it)
actually have a default for parallel
pass id to after.batch too
wrap events run in thread
call batch hooks in serial too
be consistent
don't join workers, just their shutting down

I don’t actually know why we can’t join on the workers, but they don’t
seem to be shutting down properly after their function ends
limit number of workers to number of features run
avoid some GC issues with the queues
kill/join workers after we've pulled all results
note to self
no need to print those
Merge pull request #4 from Spentry/batch-hooks

Batch hooks