Implemented timeouts for fdset API.

src/common/fdset.h

src/common/fdset_epoll.c

src/common/fdset_epoll.h

src/common/fdset_kqueue.c

src/common/fdset_kqueue.h

src/common/fdset_poll.c

src/common/fdset_poll.h

src/knot/server/tcp-handler.c

src/knot/server/xfr-handler.c

src/tests/common/fdset_tests.c