Lock signature of signed entity types during warm-up

[jpraynaud]

Why

During the warm-up phase of the signer/aggregator we need to have the associated signed entity type locked and not attempting to create signatures.

What

Implement a lock mechanism which prevents the signed entity type being warmed-up to be signed when a lock is in place.

How

• [x] Implement a lock mechanism in common
• [x] Use the lock in the aggregator to block open message generation for the associated signed entity type
• [x] Use the lock in the signer to block the signature for the associated signed entity type