v2.1: rolls out chained Merkle shreds to ~5% of mainnet slots (backpo…

…rt of #4019) (#4131)

* rolls out chained Merkle shreds to ~5% of mainnet slots (#4019)

(cherry picked from commit d3fe8f8)

# Conflicts:
#	turbine/src/broadcast_stage/standard_broadcast_run.rs

* resolves merge conflicts

---------

Co-authored-by: behzad nouri <behzadnouri@gmail.com>

turbine/src/broadcast_stage/standard_broadcast_run.rs

@@ -503,12 +503,13 @@ impl BroadcastRun for StandardBroadcastRun {
     }
 }
 
-fn should_chain_merkle_shreds(_slot: Slot, cluster_type: ClusterType) -> bool {
+fn should_chain_merkle_shreds(slot: Slot, cluster_type: ClusterType) -> bool {
     match cluster_type {
         ClusterType::Development => true,
         ClusterType::Devnet => true,
-        ClusterType::MainnetBeta => false,
-        ClusterType::Testnet => false,
+        // Roll out chained Merkle shreds to ~5% of mainnet slots.
+        ClusterType::MainnetBeta => slot % 19 == 1,
+        ClusterType::Testnet => true,
     }
 }