refine the implementation of IndexMergeReader

Feature Request

Is your feature request related to a problem? Please describe:

The implementation of IndexMergeReader is rough now, we can refine it further to make the code more readable and correct.

Describe the feature you'd like:

• Remove the useless attribute KeepOrder for IndexMergeReaderExecutor. IndexMerge does not promise the output order and does not need the order of its children now, thus we do not need the KeepOrder attribute now. issue: https://github.com/pingcap/tidb/issues/14339
• Remove the attribute indexMergeProcessWorker.maps, we can define this variable as a local variable in indexMergeProcessWorker.fetchLoop. issue: https://github.com/pingcap/tidb/issues/14339
• Refine the IPC between partialWorker and 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 issue: https://github.com/pingcap/tidb/issues/14333
• Runtime stats and feedback are not handled correctly during indexMergeReaderExecutor, we need to make them work. https://github.com/pingcap/tidb/issues/14334
• We can use WithRecovery instead of the way used to recover to make the code more readable. Refer this. issue: https://github.com/pingcap/tidb/issues/14335
• Start all the workers for the first time Next is called instead Open. issue: https://github.com/pingcap/tidb/issues/14336
• Send the result chk back to indexMergeProcessWorker to save the memory usage. Refer this issue: https://github.com/pingcap/tidb/issues/14337
• Send a batch of handles to the indexMergeTableScanWorkers instead of the handles of a task which is filtered by indexMergeProcessWorker.maps. issue: https://github.com/pingcap/tidb/issues/14338