Trie: Add support for unbound subnet walks

Trie: Add support for unbound subnet walks

Trie walk supports adding restriction on walk bounds by supplying option net
during walk initialization. Currently, walk initialized this way couldn't escape
specified subnet. By additional flag, we add support to drop to subnet's upper
bound for subnet started walks. Therefore it is now possible to start a walk in
some subnet and continue walking all lexicagraphical successors present in the
trie.