`feature_service` provided two sets of features: `known_feature_set` and
`supported_feature_set`. The purpose of both and the distinction between
them was unclear and undocumented.
The 'supported' features were gossiped by every node. Once a feature is
supported by every node in the cluster, it becomes 'enabled'. This means
that whatever piece of functionality is covered by the feature, it can
by used by the cluster from now on.
The 'known' set was used to perform feature checks on node start; if the
node saw that a feature is enabled in the cluster, but the node does not
'know' the feature, it would refuse to start. However, if the feature
was 'known', but wasn't 'supported', the node would not complain. This
means that we could in theory allow the following scenario:
1. all nodes support feature X.
2. X becomes enabled in the cluster.
3. the user changes the configuration of some node so feature X will
become unsupported but still known.
4. The node restarts without error.
So now we have a feature X which is enabled in the cluster, but not
every node supports it. That does not make sense.
It is not clear whether it was accidental or purposeful that we used the
'known' set instead of the 'supported' set to perform the feature check.
What I think is clear, is that having two sets makes the entire thing
unnecessarily complicated and hard to think about.
Fortunately, at the base to which this patch is applied, the sets are
always the same. So we can easily get rid of one of them.
I decided that the name which should stay is 'supported', I think it's
more specific than 'known' and it matches the name of the corresponding
gossiper application state.
Closes#11512
2fe3e67a47