Removed skip-list structure.

Knot.files

src/Makefile.am

src/common/skip-list.c

src/common/skip-list.h

src/knot/server/tcp-handler.c

src/knot/server/xfr-handler.h

src/libknot/zone/node.c

src/libknot/zone/node.h

src/tests/Makefile.am

src/tests/common/skiplist_tests.c

src/tests/common/skiplist_tests.h

src/tests/unittests_main.c