[More Research Required] Support Locked Funds

[patrick-ogrady]

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.