Certificate pending route overwhelms the Cardano node in aggregator

[jpraynaud]

Why

The Cardano node suffers socket resource exhaustion as explained in the #1803. After investigation, it appears that the cause of the exhaustion is that the pending certificate route of the aggregator is making a call to the Chain Observer to retrieve some information since PR #1601.

What

Remove the call to the Chain Observer and use information from the pending certificate instead

How

• Remove the call to the Chain Observer to retrieve the current time point in the aggregator route
• Make beacon field optional in CertificatePendingMessage of Open API
• Benchmark performance of the pending certificate route (before/after)