feat: create blob info snapshot checkpoints at the epoch boundary

[shuowang12]

When enabled via the new blob_info_snapshot config section (off by default), the storage node creates a RocksDB checkpoint of its database at the epoch boundary, directly after garbage-collection phase 1 and before any further events are processed, and removes older checkpoints so at most one exists at a time. Startup finishes any cleanup that was interrupted by a crash, so a stale checkpoint cannot pin deleted SST files for a full epoch.

At that boundary point the blob info tables are identical across all honest nodes, so the checkpoints allow verifying blob info determinism across nodes offline. Checkpoint errors are logged and counted but never fail epoch processing.

Description

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Test plan

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[github-actions]

Warning: This PR modifies one of the example config files. Please consider the
following:

• Make sure the changes are backwards compatible with the current configuration.
• Make sure any added parameters follow the conventions of the existing parameters; in
  particular, durations should take seconds or milliseconds using the naming convention
  _secs or _millis, respectively.
• If there are added optional parameter sections, it should be possible to specify them
  partially. A useful pattern there is to implement Default for the struct and derive
  #[serde(default)] on it, see BlobRecoveryConfig as an example.
• You may need to update the documentation to reflect the changes.