Cluster-specific protocol should not be included in Core's protocol

[lolski]

Problem to Solve

[This issue has been generalised based on Alex's message below]

The protocol spec includes definitions specific to Core and Cluster in one place. However, this exposes Cluster APIs on the Core server that could accidentally be implemented and additionally represents a domain leak. The Cluster protocol should be separate and extend the Core protocol in a separately released package.

[alexjpwalker]

read_any_replica is now part of the Options message, however the general message of this issue (it's visible to Core clients) still applies.

[flyingsilverfin]

Note: since we are planning on absorbing the protocol into the server repositories, this split should occur naturally.