Vnodes-to-tablets migration needs a finalization step to finish or
rollback the migration. Finishing the migration involves switching the
keyspace schema to tablets and clearing the `intended_storage_mode` from
system.topology. Rolling back the migration involves deleting the tablet
maps and clearing the `intended_storage_mode`.
The finalization needs to be done as a topology request to exclude with
other operations such as repair and TRUNCATE.
This patch introduces the `finalize_migration` global topology request
for this purpose. The request takes a keyspace name as an argument.
The direction of the finalization (i.e., forward path vs rollback) is
inferred from the `intended_storage_mode` of all nodes (not ideal,
should be made explicit).
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>
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