Inline keepalive packet support for BFD

Multihop BFD inline keepalive support enables scaling up to 10 inline BFD sessions with 150-millisecond support on both multihop BFD sessions as well as single-hop inline sessions. Multihop BFD session intervals can also be configured to less than one-second granularity. This enables both faster detection of link failures and recovery. The switch will also send keepalive messages according to the configured interval.