Malachi.
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A ra (Raft) state machine that replicates Malachi.Cluster.ClusterFlags, the cluster's
operator-enabled feature flags.
apply/3 delegates to the pure, deterministic ClusterFlags.apply/2, so the flags become
Raft-replicated with no logic in the machine itself, exactly the shape of
Malachi.Cluster.RingMachine and Malachi.Cluster.LeaseMachine. Like the ring it needs no clock:
switching a flag on is a set insert.
Versioned through Malachi.Cluster.MachineVersion, which is also what keeps a member running a build
from before this store existed from ever being told to enable a flag: {:enable_flag, 2} was
introduced at machine version 2, so it is refused identically on every member until the last one runs
a build that supports 2.