fix: cleanup slots on non-fresh ledger (#404)

On start of `LedgerTruncator` we initialize lowest_cleaup_slot

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## Greptile Summary

Added initialization of lowest_cleanup_slot in LedgerTruncator to
prevent unintended truncation when starting with non-fresh ledgers.

- Added new method `initialize_lowest_cleanup_slot` in
`magicblock-ledger/src/store/api.rs` to scan blockhash CF for lowest
available slot
- Modified LedgerTruncator startup in
`magicblock-ledger/src/ledger_truncator.rs` to properly initialize
lowest cleanup slot
- Default lowest cleanup slot to 0 if no slots are found in blockhash
column family



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Author: taco-paco

magicblock-ledger/src/ledger_truncator.rs

@@ -47,6 +47,9 @@ impl<T: FinalityProvider> LedgerTrunctationWorker<T> {
     }
 
     pub async fn run(self) {
+        self.ledger
+            .initialize_lowest_cleanup_slot()
+            .expect("Lowest cleanup slot initialization");
         let mut interval = interval(self.truncation_time_interval);
         loop {
             tokio::select! {

magicblock-ledger/src/store/api.rs

@@ -240,6 +240,26 @@ impl Ledger {
             .expect(Self::LOWEST_CLEANUP_SLOT_POISONED)
     }
 
+    /// Initializes lowest slot to cleanup from
+    pub fn initialize_lowest_cleanup_slot(&self) -> Result<(), LedgerError> {
+        match self.blockhash_cf.iter(IteratorMode::Start)?.next() {
+            Some((lowest_slot, _)) => {
+                *self
+                    .lowest_cleanup_slot
+                    .write()
+                    .expect(Self::LOWEST_CLEANUP_SLOT_POISONED) = lowest_slot;
+            }
+            None => {
+                *self
+                    .lowest_cleanup_slot
+                    .write()
+                    .expect(Self::LOWEST_CLEANUP_SLOT_POISONED) = 0;
+            }
+        }
+
+        Ok(())
+    }
+
     // -----------------
     // Block time
     // -----------------