Ledger cleanup fixes (#9131)

* Fix purging happening every slot when cleanup service is not started at slot 0
* Purge by shred count instead of slots since slots can have variable
number of shreds

ledger/src/blockstore_db.rs

@@ -53,7 +53,7 @@ pub enum BlockstoreError {
     FsExtraError(#[from] fs_extra::error::Error),
     SlotCleanedUp,
 }
-pub(crate) type Result<T> = std::result::Result<T, BlockstoreError>;
+pub type Result<T> = std::result::Result<T, BlockstoreError>;
 
 impl std::fmt::Display for BlockstoreError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {