Settle execution task after all tasks are completed

This commit prevents different goroutines from reading the same statedb at the same time, resulting a concurrent read/write panic from trie tracer.

core/blockstm/executor.go

@@ -376,16 +376,6 @@ func (pe *ParallelExecutor) Prepare() error {
 		}(i)
 	}
 
-	pe.settleWg.Add(1)
-
-	go func() {
-		for t := range pe.chSettle {
-			pe.tasks[t].Settle()
-		}
-
-		pe.settleWg.Done()
-	}()
-
 	// bootstrap first execution
 	tx := pe.execTasks.takeNextPending()
 

core/parallel_state_processor.go

@@ -408,6 +408,10 @@ func (p *ParallelStateProcessor) Process(block *types.Block, statedb *state.Stat
 		return nil, nil, 0, err
 	}
 
+	for _, task := range tasks {
+		task.Settle()
+	}
+
 	// Finalize the block, applying any consensus engine specific extras (e.g. block rewards)
 	p.engine.Finalize(p.bc, header, statedb, block.Transactions(), block.Uncles(), nil)