To improve the behavior of the removenode operation, we want to issue
a global topology barrier after the removenode has been applied.
However, this requires changing the topology state machine to add a new
state (left_token_ring) to the removenode flow, which is not supported
by older nodes.
To allow rolling upgrades, we add a feature flag
REMOVENODE_WITH_LEFT_TOKEN_RING that controls whether the new removenode
flow is used.
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