Refactor block/tx sanity checks to use txscript.ScriptFlags for feature toggling

[tyler-smith]

PR #51 introduced a boolean flag for whether or not the magnetic anomaly hardfork is active for the purposes of counting sigops.

A better solution is to use the existing ScriptFlags defined in the txscript subpackage. This will allow us to represent all potentially toggled features with a single object instead of passing around a ton of booleans in the future.

After looking at the ABC code, they do pretty much the same thing with their BlockchainValidationOptions type: https://github.com/Bitcoin-ABC/bitcoin-abc/blob/efa9749bac3e20b2a8fc056c01d75e3dbc4e341d/src/validation.cpp#L3025

[zquestz]

I already started this work. It actually isn't too bad =)

[zquestz]

Done