Refactor `dns_rdataset_t->privateN`

The generic dns_rdataset_t structure has a number of private fields that are (according to the comment before their declarations) "for use by the rdataset implementation, and MUST NOT be changed by clients." That suggests they should only be used by dns_rdatalist_t and the rdataslab implementation (which is largely in rbtdb.c). However, the privateN fields are also used by:

• dnsrps.c
• keytable.c
• ncache.c
• sdb.c
• sdlz.c

It is not clear what each privateN field is for, which code owns which fields, whether or not there are clashes (or how developers can be sure to avoid them).

This is one of the work items tracked in #3268