Zero-token nodes must be supported by all nodes in the cluster.
Otherwise, the non-supporting nodes would crash on some assertion
that assumes only token-owing normal nodes make sense.
Hence, we introduce the ZERO_TOKEN_NODES cluster feature. Zero-token
nodes refuse to boot if it is not supported.
I tested this patch manually. First, I booted a node built in the
previous patch. Then, I tried to add a zero-token node built in this
patch. It refused to boot as expected.
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