optimize redeemscript map generation

This changes the way we're generating the redeemScripts. We used to generate all the possible redeemScripts for the max used indices, which is inefficient. This PR chances that to use a list of paths, derived from the utxos used for the tx.
I've ran the tests and all pass.
fixes https://github.com/bitpay/copay/issues/732
Can we now remove this comment?