Enhance aggregator state machine to support `Blocked` state

[jpraynaud]

Why

We want to enhance the state machine of the aggregator to better support when there is no genesis certificate yet or in case of epoch gap. This will avoid numerous logs which are not needed in these situations

What

Update the state machine of the aggregator to support blocked state for the rest of the epoch

How

• Add a new Blocked state (with a reason field which is an enum) in the aggregator state machine
• The state machine transitions out of this state upon epoch change and gets back to Idle
• In case of missing genesis go to this new state (from Idle state)
• In case of genesis epoch (genesis certificate was created during this epoch) go to this new state (from Idle state)
• In case of epoch gap, go to this new state (from Idle state)
• Add/update integration tests in aggregator
• Update documentation