[bfd] BFD HW offload for BGP sessions

[baorliu]

BGP can be configured to use BFD to monitor it's neighbor reachability. when the BFD session becomes DOWN, it indicates that BGP neighbor become not reachable, so BGP can updates the corresponding route quickly based on BFD state, the detection can be much faster than the detection using BGP keep alive approach. Currently, BGP can use frr/bfdd (a software BFD) to do this. The proposed design let BGP use hardware offloaded BFD in SONiC, it provide a faster detection, less CPU load, and supports more BGP sessions to use BFD.

Repo	PR title	State
sonic-swss[master]	BFD hardware offload for BGP sessions