[More Research Required] Support Locked Funds

In some blockchains, funds only become accessible after some future block index. We should add a field on the operation model (or on the coin model) that allows an implementer to indicate this is the case (right now this usually happens in metadata).

One very important example of this is the lock on Bitcoin `coinbase` funds (which can't be spent for 100 blocks). See `COINBASE_MATURITY` in the [Bitcoin protocol rules](https://en.bitcoin.it/wiki/Protocol_rules).