initial experiments for background workers

Knot.files

src/Makefile.am

src/knot/main.c

src/knot/worker/pool.c

src/knot/worker/pool.h

src/knot/worker/queue.c

src/knot/worker/queue.h

src/knot/worker/task.c

src/knot/worker/task.h

src/knot/zone/events.c

src/knot/zone/events.h