[Optimization] Avoid checking certificate signatures twice.

[acoglio]

As a validator collects endorsing signatures from other validators, it verifies them. When it has enough signatures, it creates a certificate with BatchCertificate::from(), which verifies the signatures again. We can avoid this duplication by having the validator call BatchCertificate::from_unchecked() instead. BatchCertificate::from() performs a few other simple checks besides verifying signatures; these other simple checks are easy for the validator to perform before calling BatchCertificate::from_unchecked().