Apply mandatory coinbase consensus check only for policyAsset

[tomt1664]

Only apply the consensus check for the con_mandatory_coinbase destination to coinbase outputs for policyAsset. This is in preparation for issued asset fee support, allowing non-policy asset (issued assets) to have custom destinations (including AMP destinations).

This is a consensus change and a technical hardfork when deployed, however as there is no existing support in Elements for either creating blocks with issued asset fees or accepting/relaying issued asset fee transactions, and can therefore be safely released and deployed.

[apoelstra]

This is a consensus change and a technical hardfork when deployed, however as there is no existing support in Elements for either creating blocks with issued asset fees or accepting/relaying issued asset fee transactions, and can therefore be safely released and deployed.

To be clearer: this is more than a "technical" hardfork. The point is that exploiting it to split the network can only be done by a quorum of blocksigners, who cannot do it by accident because their existing software does not support it (and who, by their position as blocksigners, are already trusted not to fork the network on purpose.)

Therefore, any further changes to create non-policyAsset coinbase outputs will need to be coordinated with a flag day etc. But releasing this support ASAP minimizes the chances of people missing such a flag day, if and when one is scheduled.

[apoelstra]

utACK b543259

[delta1]

utACK b543259

[apoelstra]

ACK b543259; successfully ran local tests

[apoelstra]

ACK b543259; successfully ran local tests