dedups shreds by common-header instead of entire payload

Shreds in the retransmit stage:
  * don't have repair nonce (repaired shreds are not retransmitted).
  * are already resigned by this node as the retransmitter.
  * have their leader's signature verified.
Therefore in order to dedup shreds, it suffices to compare:

    (signature, slot, shred-index, shred-type)

Because ShredCommonHeader already includes all of the above tuple,
the rest of the payload can be skipped.

ledger/src/shred.rs

@@ -638,6 +638,11 @@ pub mod layout {
         packet.buffer_mut().get_mut(..size)
     }
 
+    #[inline]
+    pub fn get_common_header_bytes(shred: &[u8]) -> Option<&[u8]> {
+        shred.get(..SIZE_OF_COMMON_SHRED_HEADER)
+    }
+
     pub(crate) fn get_signature(shred: &[u8]) -> Option<Signature> {
         shred
             .get(..SIZE_OF_SIGNATURE)

turbine/src/retransmit_stage.rs

@@ -165,10 +165,22 @@ impl<const K: usize> ShredDeduper<K> {
             .maybe_reset(rng, false_positive_rate, reset_cycle);
     }
 
+    // Returns true if the shred is duplicate and should be discarded.
+    #[must_use]
     fn dedup(&self, key: ShredId, shred: &[u8], max_duplicate_count: usize) -> bool {
+        // Shreds in the retransmit stage:
+        //   * don't have repair nonce (repaired shreds are not retransmitted).
+        //   * are already resigned by this node as the retransmitter.
+        //   * have their leader's signature verified.
+        // Therefore in order to dedup shreds, it suffices to compare:
+        //    (signature, slot, shred-index, shred-type)
+        // Because ShredCommonHeader already includes all of the above tuple,
+        // the rest of the payload can be skipped.
         // In order to detect duplicate blocks across cluster, we retransmit
         // max_duplicate_count different shreds for each ShredId.
-        self.deduper.dedup(shred)
+        shred::layout::get_common_header_bytes(shred)
+            .map(|header| self.deduper.dedup(header))
+            .unwrap_or(true)
             || (0..max_duplicate_count).all(|i| self.shred_id_filter.dedup(&(key, i)))
     }
 }