Cancels an ongoing negotiation on a proposal. This does not cancel or end serving for the deals if the proposal has been finalized. If the proposal has not been finalized before, calling this method will set the Proposal.state to TERMINATED and increment the Proposal.proposal_revision. If the proposal has been finalized before and is under renegotiation now, calling this method will reset the Proposal.state to FINALIZED and increment the Proposal.proposal_revision. This method does not support private auction proposals whose Proposal.deal_type is 'PRIVATE_AUCTION'.