Transaction Composition API v2

[adamkrellenstein]

The minimal responsability of the server is just to construct the data/outputs, and to order the inputs if necessary. We want to remove utxo_locks, etc. And we'll need to continue supporting the current code as v1 ofc.

We should support having the node fetch the pubkey and unspent_set from an electrs service determined by --electrs-url (defaulting to api.counterparty.io:3000); if the user passes those params in during tx construction, then no connection is made; if a user sets --electrs-url='', then those params are required for tx construction. (We could also allow fetching from a local bitcoind instance for cases where the source are in bitcoind's wallet and that service is tracking the necessary UTXOs.