Implement tiered storage

Introduces TierStore, a KVStore implementation that manages data
across three storage layers:

- Primary: Main data store for critical node data
- Ephemeral: Secondary store for non-critical, easily-rebuildable
data (e.g., network graph) with fast local access
- Backup: Tertiary store for disaster recovery with async/lazy
operations to avoid blocking primary store
- Unit tests for TierStore core functionality

Author: enigbe

src/io/mod.rs

@@ -10,6 +10,7 @@
 pub mod sqlite_store;
 #[cfg(test)]
 pub(crate) mod test_utils;
+pub(crate) mod tier_store;
 pub(crate) mod utils;
 pub mod vss_store;