Avoid netievent allocations when the callbacks can be called directly

Avoid netievent allocations when the callbacks can be called directly

After turning the users callbacks to be asynchronous, there was a
visible performance drop.  This commit prevents the unnecessary
allocations while keeping the code paths same for both asynchronous and
synchronous calls.

The same change was done to the isc__nm_udp_{read,send} as those two
functions are in the hot path.