These are arbitrary numbers, but that's what C# node uses. maxRetries could be
set lower because we have exponential backoff anyway, but this works too.
And don't add/resend it multiple times.
1. We can be in a setup with one SV only and no AddSignature() called at all.
2. AddSignature() might add M-1 signatures and our signature should be the
last one to complete MPTRoot, but we'll never do that.
It's only used to sign/verify it and is not a part of the structure. It's
still neded in consensus.Payload though because that's the way dbft library
is.
