[BFD]BFD V0.3 Proposal

[zhichuanhe]

I have discussed the proposal with @ragu-raman through email,and the detail information refer to the SAI_Change_Proposal_BFD_v0_3.docx. V0.3 Change : 1.Encapsulate the BFD packet more flexible by next hop 2.Add option for timer negotiation (P/F process) 3.Add option for remote discriminator learning