fix: Check proof block numbers re committed and proven tips

[sergerad]

Summary

Adds block number checks to the ProofSync::sync() and State::apply_proof() functions to ensure that:

1. Proofs are applied in ascending order; and
2. Proofs are applied only for already-committed blocks.

Adds an assert to ProvenTipWriter::advance().

Adds a stub verify block proof fn and uses it in apply_proof().

Adds a check for already validated transactions in SubmitProvenTransaction handler.

Changelog

changelog = "none"
reason    = "Internal change only."

[sergerad]

@Mirko-von-Leipzig should we enable TLS for the streams by default and allow for a flag to disable?