A replica's read-only flag in the volume layout only moved on registration
and on volume.mark. A change that arrived in the regular heartbeat updated
the node's record, which the writable list follows, but not the layout
flag, which the vacuum sweep reads. So the sweep kept trying volumes on a
disk that had gone read-only while the server ran, and after a restart it
skipped volumes that had since become writable again until the next
restart (issue #11516).
Apply the reported state to the flag for every changed volume. Only the
flag: the writable list stays with EnsureCorrectWritables and its
capacity guards.
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