Prevent reconstruction starting prematurely (#6669)

* Prevent reconstruction starting prematurely

* Simplify condition

* Merge remote-tracking branch 'origin/release-v6.0.1' into dont-start-reconstruction-early

Author: michaelsproul

beacon_node/beacon_chain/src/builder.rs

@@ -1037,7 +1037,9 @@ where
         );
 
         // Check for states to reconstruct (in the background).
-        if beacon_chain.config.reconstruct_historic_states {
+        if beacon_chain.config.reconstruct_historic_states
+            && beacon_chain.store.get_oldest_block_slot() == 0
+        {
             beacon_chain.store_migrator.process_reconstruction();
         }