Draft: Refactor taskmgr to run on top of netmgr (v9.16)

This commit changes the taskmgr to run the individual tasks on the netmgr internal workers. While an effort has been put into keeping the taskmgr interface intact, couple of changes have been made:

• The taskmgr has no concept of universal privileged mode - rather the tasks are either privileged or unprivileged (normal). The privileged tasks are run as a first thing when the netmgr is unpaused. There are now four different queues in in the netmgr:

  1. priority queue - netievent on the priority queue are run even when the taskmgr enter exclusive mode and netmgr is paused. This is needed to properly start listening on the interfaces, free resources and resume.

  2. privileged task queue - only privileged tasks are queued here and this is the first queue that gets processed when network manager is unpaused using isc_nm_resume(). All netmgr workers need to clean the privileged task queue before they all proceed normal operation. Both task queues are processed when the workers are finished.

  3. task queue - only (traditional) task are scheduled here and this queue along with privileged task queues are process when the netmgr workers are finishing. This is needed to process the task shutdown events.

  4. normal queue - this is the queue with netmgr events, e.g. reading, sending, callbacks and pretty much everything is processed here.

• The isc_taskmgr_create() now requires initialized netmgr (isc_nm_t) object.

• The isc_nm_destroy() function now waits for indefinite time, but it will print out the active objects when in tracing mode (-DNETMGR_TRACE=1 and -DNETMGR_TRACE_VERBOSE=1), the netmgr has been made a little bit more asynchronous and it might take longer time to shutdown all the active networking connections.

• Previously, the isc_nm_stoplistening() was a synchronous operation. This has been changed and the isc_nm_stoplistening() just schedules the child sockets to stop listening and exits. This was needed to prevent a deadlock as the the (traditional) tasks are now executed on the netmgr threads.

• The socket selection logic in isc__nm_udp_send() was flawed, but fortunatelly, it was broken, so we never hit the problem where we created uvreq_t on a socket from nmhandle_t, but then a different socket could be picked up and then we were trying to run the send callback on a socket that had different threadid than currently running.

(cherry picked from commit b540722b)

Closes #2638 (closed)