refine the IPC in indexMergeReaderExecutor

[XuHuaiyu]

indexMergeProcessWorker. indexMergeProcessWorker.fetchLoop use partialWorkerCnt-- to check whether all the partialWorkers finishing their work. We can use a WaitGroup to synchronize the work between all the partial workers, and start a goroutine which waits for the WaitGroup, and close the fetchCh when all the partialWorkers exit. When indexMergeProcessWorker fails to fetch data from fetchCh, it knows all the partialWorkers exit. Refer this.