lib: optimise prefix list setup

lib: optimise prefix list setup

- duplicate prefix check can use the trie structure
- appending with a seq# beyond the end of the list can shortcut

Configuration load is now bottlenecked by cmd_element_match() and
strcmp().  For a real-world routeserver prefix list configuration
(38668 lines of config for multiple prefix lists):
  before: 4.73s
  after:  1.92s    x 2.46

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>