The verbs are:
* DEFINITIONS_UPDATE (push)
* MIGRATION_REQUEST (pull)
Support was added in a backward-compatible way. The push verb, sends
both the old frozen mutation parameter, and the new optional canonical
mutation parameter. It is expected that new nodes will use the latter,
while old nodes will fall-back to the former. The pull verb has a new
optional `options` parameter, which for now contains a single flag:
`remote_supports_canonical_mutation_retval`. This flag, if set, means
that the remote node supports the new canonical mutation return value,
thus the old frozen mutations return value can be left empty.
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