This is a boyscouting commit to standardize the threading of zone fills.
We do not need to join threads after their completion, instead we simply
allow them to clean up their memory without blocking the user. This
also sets the maximum number of threads that may be created to the
number of zones being filled. More than this will only leave un-used
threads being created and immediately killed.
We also include the connectivity search as a phase in the fill progress
reporter. This was the case before but did not utilize the correct
maxsize, leading to stalled progress bar.
Seth Hillbrand
Jeff Young
Tomasz Włostowski