optimize txp protocol

This PR is part of an ongoing process of securing copay against peers faking txp metadata.

The previous version of the TxProposals protocol sent an array containing exactly 1 TxProposal. (See https://github.com/bitpay/copay/blob/master/js/models/core/Wallet.js#L120), but all the merge process assumed it would be a variable length array. This PR removes the array by only sending a single TxProposal, and refactors all the code to handle this simpler case (merging an array of tx proposals was much more complex than handling one at a time)