Allow Mirrors to be promoted to primary

[ripienaar]

Proposed change

We need to formalise how a Mirror becomes the primary in a pair of streams.

Today we do not support removing the Mirror configuration from a stream, at a basic level we should allow that.

More difficult though is the entire workflow and how the old origin can somehow become a mirror again without entirely resyncing the entire stream.

This also implies we need some way to add Mirror to a running stream under some quite strict circumstances

Use case

Failover scenarios enabled by mirrors

Contribution

No response