pctrie: create iterator

Define a pctrie iterator type. A pctrie iterator is a wrapper around a
pctrie that remembers a position in the trie where the last search
left off, and where a new search can resume. When the next search is
for an item very near in the trie to where the last search left off,
iter-based search is faster because instead of starting from the root,
the search usually only has to back up one or two steps up the
root-to-last-search path to find the branch that leads to the new
search target.

Every kind of lookup (plain, lookup_ge, lookup_le) that can begin with
the trie root can begin with an iterator instead. An iterator can also
do a relative search ("look for the item 4 greater than the last item
I found") because it remembers where that last search ended. It can
also search within limits ("look for the item bigger than this one,
but it has to be less than 100"), which can save time when the next
item beyond the limits and that is known before we actually know what
that item it is. An iterator can also be used to remove an item that
has already been found, without having to search for it again.

Iterators are vulnerable to unsynchronized data changes. If the
iterator is created with a lock held, and that lock is released and
acquired again, there's no guarantee that the iterator path remains
valid.

Reviewed by: markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D45627