retains chained Merkle root across leader slots (#1057)

Looking up Merkle root of the last erasure batch for the parent block
can fail if the slot-meta is not yet available in blockstore.
This commit instead retains chained Merkle root across leader slots. If
the parent of the current block is the previous leader slot, then the
chained Merkle root is readily available and we can bypass blockstore
lookup.

turbine/src/broadcast_stage.rs

@@ -66,6 +66,8 @@ pub enum Error {
     Blockstore(#[from] solana_ledger::blockstore::BlockstoreError),
     #[error(transparent)]
     ClusterInfo(#[from] solana_gossip::cluster_info::ClusterInfoError),
+    #[error("Duplicate slot broadcast: {0}")]
+    DuplicateSlotBroadcast(Slot),
     #[error("Invalid Merkle root, slot: {slot}, index: {index}")]
     InvalidMerkleRoot { slot: Slot, index: u64 },
     #[error(transparent)]

turbine/src/broadcast_stage/broadcast_utils.rs

@@ -26,15 +26,6 @@ pub(super) struct ReceiveResults {
     pub last_tick_height: u64,
 }
 
-#[derive(Clone)]
-pub struct UnfinishedSlotInfo {
-    pub(super) chained_merkle_root: Hash,
-    pub next_shred_index: u32,
-    pub(crate) next_code_index: u32,
-    pub slot: Slot,
-    pub parent: Slot,
-}
-
 pub(super) fn recv_slot_entries(receiver: &Receiver<WorkingBankEntry>) -> Result<ReceiveResults> {
     let target_serialized_batch_byte_count: u64 =
         32 * ShredData::capacity(/*merkle_proof_size*/ None).unwrap() as u64;