Implemented ixfr-from-differences in config.

samples/knot.full.conf

src/knot/conf/cf-lex.l

src/knot/conf/cf-parse.y

src/knot/conf/conf.c

src/knot/conf/conf.h

src/knot/server/zones.c

src/libknot/updates/changesets.c

src/libknot/zone/zone-diff.c