lb: add source ip based sticky load balancing

lb: add source ip based sticky load balancing

This patch adds source ip based sticky session, which is already
implemented in many hardware LBs and software LBs. Note that sticky
sessions may be reset if the hash is recalculated as ASs are added
or deleted.

Since this feature is unrelated to the other existing options, the
lb_add_del_vip API version has been upgraded to v2 and a new option
"src_ip_sticky" has been added.

Type: feature
Signed-off-by: Nobuhiro MIKI <nmiki@yahoo-corp.jp>
Change-Id: I3eb3680a28defbc701f28c873933ec2fb54544ab