use a qp-trie for the forwarders table

use a qp-trie for the forwarders table

Instead of an RBT for the forwarders table, use a QP trie.

We now use reference counting for dns_forwarders_t. When a forwarders
object is retrieved by dns_fwdtable_find(), it must now be explicitly
detached by the caller afterward.

QP tries require stored objects to include their names, so the
the forwarders object now has that. This obviates the need to
pass back a separate 'foundname' value from dns_fwdtable_find().