Migrate existing cache values into ephemeral storage

Nodes adopting tiered storage may already hold cache values in primary storage,
while a missing index leaves existing ephemeral values without a recoverable
position in the cross-store ordering.

For this commit, we:
- Prepare namespaces on reads as well as writes and listings so first access
cannot bypass migration or journal recovery.

- Rebuild missing indexes from primary ordering, discard ephemeral values whose
ordering cannot be recovered, and move indexed cache values to ephemeral storage
without changing their index positions.

- Copy values before removing primary and backup copies, and persist a completion
marker so interrupted reconciliation can safely resume without repeating it on
every access.

- Preserve namespace-specific cache routing for future namespace
changes.

Author: enigbe