Split fast and slow task queues

Split fast and slow task queues

Change the taskmgr (and thus netmgr) in a way that it supports fast and
slow task queues.  The fast queue is used for incoming DNS traffic and
it will pass the processing to the slow queue for sending outgoing DNS
messages and processing resolver messages.

In the future, more tasks might get moved to the slow queues, so the
cached and authoritative DNS traffic can be handled without being slowed
down by operations that take longer time to process.

(cherry picked from commit 1b3b0cef224e7a9e8279c5cfe2f7e188e3777cc7)