Clarify reference counting in RBTDB database

Clarify reference counting in RBTDB database

Change the names of the node reference counting functions
and add comments to make the mechanism easier to understand:

- new_reference() and decrement_references() are now called
  dns__rbtnode_acquire() and dns__rbtnode_release()
  respectively; this reflects the fact that they modify both
  the internal and external reference counters for a node.

- rbtnode_newref() and rbtnode_decref are now called
  rbtnode_erefs_increment() and rbtnode_erefs_decrement(),
  to reflect that they only increase and decrease the node's
  external reference counters, not internal.

(cherry picked from commit 857225aeb65497f979e459269f6425f88f0e3cd2)