Unify the approaches of spawning background jobs

[ekexium]

In the current codebase, client-go provides several ways to spawn background or asynchronous tasks:

• builtin go
• kvstore.Go(func())
• KVTxn.spawnWithStorePool

Using multiple approaches simultaneously may lead to issues such as data races (as seen in pingcap/tidb#57213).

We should standardize on a single approach to minimize risks and maintain code simplicity.