refactor the slab rdataset implementation

refactor the slab rdataset implementation

- use externally accessible functions for attachnode/detachnode
  so these functions can be moved outside rbtdb.c
- simplify and tidy up some other functions
- use struct initializers when appropriate
- remove the flag RDATASET_ATTR_RETAIN; it was never being set
- renamed the rdataset attributes to
- remove the 'slab_methods' rdataset implementation. this was
  a reduced set of slab rdataset methods, omitting 'setownercase()'
  and 'getownercase()'. we can get the identical result by using
  an DNS_RDATASETATTR_KEEPCASE attribute in rdatasets that
  shouldn't have their case modified, and then we only need one
  set of rdataset methods.