Clarify reference counting in QP databases

Clarify reference counting in QP databases

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

- newref() and decref() are now called qpcnode_acquire()/
  qpznode_acquire() and qpcnode_release()/qpznode_release()
  respectively; this reflects the fact that they modify both
  the internal and external reference counters for a node.

- qpcnode_newref() and qpznode_newref() are now called
  qpcnode_erefs_increment() and qpznode_erefs_increment(), and
  qpcnode_decref() and qpznode_decref() are now called
  qpcnode_erefs_decrement() and qpznode_erefs_decrement(),
  to reflect that they only increase and decrease the node's
  external reference counters, not internal.