siftr: three changes that improve performance

Summary:
(1) use inp_flowid or a new packet hash for a flow identification
(2) cache constant connection info into struct flow_hash_node
(3) use compressed notation for IPv6 address representation

Reviewers: rscheff, tuexen
Approved by: tuexen (mentor)
Subscribers: imp, melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D40302