A0-2846: aleph-bft shouldn't quit after creator's graceful exit

[fixxxedpoint]

Description

Creator of aleph-bft might decide to gracefully exit when it discovers inconsistency of its units backup compared to knowledge collected from network. In that case, aleph-bft's session should be allowed to continue instead of panicking.

Type of change

• New feature (non-breaking change which adds functionality): allow to disable creator
• Fix: channel returned by Terminator was already of type FusedStream. When used with futures::select! it could cause that select to panic, since its implementation returns is_terminated() == true before it return an error (after we drop parents Terminator).

[github-actions[bot]]

Please make sure the following happened

• [ ] Appropriate tests created
• [ ] Infrastructure updated accordingly
• [ ] Updated existing documentation
• [ ] New documentation created
• [ ] Version bumped if breaking changes