support the extension of the consensus module

[GlenDC]

Currently modules (and chains build using Go-rivine) can only subscribe to the consensus module, but cannot extend it. Meaning that:

• the pre-check validation of transaction inner-block dependencies aren't checked for non-standard purposes;
• the actual validation and addition of a block is also doing as a whole prior to ever being send to a subscriber of the consensus module, meaning even for the actual addition of that block, the inner relationships of multiple Tx's within a same block are not tested for non-standard purposes at that point either;

This is an issue in tfchain currently as this blockchain has all kind of non-standard transaction versions (minting of coins and registration/management of 3bot records), for which they currently cannot test those inner-block Tx's dependencies. Should they be able to extend the consensus module logic, it would fix their issues. See threefoldfoundation/tfchain#195 for more context.

[GlenDC]

For now I leave this on state_question. So far I did not find a nice way to allow this feature. Again, it's one of those things that really shouldn't have to be solved, and only arises because the weird way we try to make use of this codebase.

Either way, will continue to figure it out and work on this once the 3bot story works on devnet of tfchain with all its features.