More IS refactoring

[UdjinM6]

No description provided.

[crowning-]

Nit: I'm used to have boolean methods named "is_" or "has_", e.g. vote.IsSignatureValid() instead of vote.CheckSignature().
You already did this with IsAllVotesValid(), so I think this would be more consistent naming wise.

[UdjinM6]

@crowning- Yeah, we actually have all variants of it in code - CheckSignature, VerifySignature and SignatureValid in different modules. (And even IsSignatureValid itself which actually should be IsScriptSigValid, lol, I just realized that, will fix :) ). There is VerifySignature in original bitcoin code which also returns bool but it takes signature as an argument (and it is closer to VerifyMessage in that way). And then there is CheckSignature in alerts module which also returns bool and verifies signature it already has (and in that sense it's much closer to what we do imo) but it has pubkey as an argument. Furthermore, we are already using CheckSignature in PS and sporks. So after going through all of this I though that CheckSignature would be a good candidate actually.

PS. Should have written all this in PR description initially but thanks for bringing the question up :)

[schinzelh]

Yeah, the codebase is not consistent re. this.
utACK 149f27a anyway :)