Coalesce warnings for deprecated lifecycle hooks and print single message after reconciliation (rather than one per component-type).

Relates to PR #12028 and [this comment](https://github.com/facebook/react/pull/12028#discussion_r162420575).
} else {
            returnFiber.firstEffect = workInProgress;
          }
          returnFiber.lastEffect = workInProgress;
        }
      }
      stopWorkTimer(workInProgress);
      if (__DEV__ && ReactFiberInstrumentation.debugTool) {
        ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);
      }
      if (siblingFiber !== null) {
        
        return siblingFiber;
      } else if (returnFiber !== null) {
        
        workInProgress = returnFiber;
        continue;
      } else {
        
        const root: FiberRoot = workInProgress.stateNode;
        root.isReadyForCommit = true;
        return null;
      }
    }
    
    
    
    return null;
  }
  function performUnitOfWork(workInProgress: Fiber): Fiber | null {