With taskmgr running on top of netmgr, the ordering of how the tasks and
netmgr shutdown interacts was wrong as previously isc_taskmgr_destroy()
was waiting until all tasks were properly shutdown and detached. This
responsibility was moved to netmgr, so we now need to do the following:
1. shutdown all the tasks - this schedules all shutdown events onto
the netmgr queue
2. shutdown the netmgr - this also makes sure all the tasks and
events are properly executed
3. Shutdown the taskmgr - this now waits for all the tasks to finish
running before returning
4. Shutdown the netmgr - this call waits for all the netmgr netievents
to finish before returning
This solves the race when the taskmgr object would be destroyed before
all the tasks were finished running in the netmgr loops.
Previously, netmgr, taskmgr, timermgr and socketmgr all had their own
isc_<*>mgr_create() and isc_<*>mgr_destroy() functions. The new
isc_managers_create() and isc_managers_destroy() fold all four into a
single function and makes sure the objects are created and destroy in
correct order.
Especially now, when taskmgr runs on top of netmgr, the correct order is
important and when the code was duplicated at many places it's easy to
make mistake.
The former isc_<*>mgr_create() and isc_<*>mgr_destroy() functions were
made private and a single call to isc_managers_create() and
isc_managers_destroy() is required at the program startup / shutdown.
