Don't block cluster upgrades on API additions only

[stgraber]

Currently when a clustered server is upgraded, it will wait for all other servers to match its exact setup if it detects that it's ahead of the cluster on either the database schema or the API version (count of API extensions).

Now this is absolutely critical for the database case as we can't have servers accessing a DB schema they can't handle, but the same isn't true for API extensions as those are used to indicate new API features or config keys that are available.

Instead what we should do is fetch the highest common API extension number for the cluster and use that to truncate the API extension list exposed to clients. Basically having a cluster report the API extension list of its oldest (by version) member.

The cluster member status for any server that's behind should reflect that it needs immediate upgrading though.