Both retentionPeriods move to deploy.<store>.retentionPeriod. Measured
before moving: each is read exactly once, in the container definition of
the module that deploys the store, and no client hive consults either --
so they are options on the auto-deployed service rather than something
every host has to agree on.
What stays in swarm.* is what a client needs to reach the store: the
package, the name it answers on, the port. Both modules said "what stays
here is its package, domain, retention and wiring" in their options-block
comments; retention no longer does, so those say so.
Renames go in deploy.nix with the rest, so the whole move keeps one home
and one file to delete when the deprecation window closes.
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