Fix ReplicatedAccessStorage shutdown without startup

[kmichel-aiven]

If ReplicatedAccessStorage startup was not executed or if it failed before completing (for instance when ZooKeeper was not configured), its destructor would call shutdown and try to join a missing thread.

Changelog category (leave one):

• Bug Fix (user-visible misbehaviour in official stable or prestable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Fix replicated access storage not shutting down cleanly when misconfigured

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