Make simple getter/setter-like functions static inline

src/knot/zone/node.c

src/knot/zone/node.h

src/knot/zone/zone-tree.c

src/knot/zone/zone-tree.h

src/libknot/rdataset.c

src/libknot/rdataset.h

src/libknot/rrset.c

src/libknot/rrset.h