Logically revert #6642 and partial #7153 to reduce unnecessary thread and concurrency process (#7318)

 The key logic behinds all these is, metrics persistence is fully asynchronous.

* The core/maxSyncOperationNum setting(added in 8.5.0) is removed due to metrics persistence is fully asynchronous.
* The core/syncThreads setting(added in 8.5.0) is removed due to metrics persistence is fully asynchronous.
* Optimization: Concurrency mode of execution stage for metrics is removed(added in 8.5.0). The only concurrency of prepare stage is meaningful and kept.
* Remove the outside preparedRequest list initialization, worker instance could always build a suitable size list in the first place (Reduce Array.copy and GC load a little).