External updater may do some preparatory work like constructing a new sstable list,
and at the end atomically replace the old list by the new one.
Decoupling the preparation from execution will give us the following benefits:
- the preparation step can now yield if needed to avoid reactor stalls, as it's
been futurized.
- the execution step will now be able to provide strong exception guarantees, as
it's now decoupled from the preparation step which can be non-exception-safe.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
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