Implement the master wallet multi-sig management logic

[itamarro]

Its general design is detailed in the deck: https://docs.google.com/presentation/d/1-8MP2yRhTy7D1YDUiauJoJ1g_ppXQbRpk0YTQ0ZFEzw/edit#slide=id.g365dfefbd4_0_5

In short, the funds in the payment zone's master bitcoin wallet are locked by the keys of all of the payment zone's validators. Managing this includes generating the master address for receiving user funds, sending funds out and signing those transactions, key management as the validator set changes, etc.

[itamarro]

So coordination of wallet creation, transaction signing, confirmation across the chains, etc is a pretty delicate matter. I've drafted out my current thoughts about how to implement this here: https://github.com/CosmicBridge/server/wiki/Validator-Set-Multi-Sig-Bitcoin-Wallet:-Wallet-Creation-and-Transaction-Coordination-Design

Any feedback or insights would be welcome.