Consolidate index build RSTL unlock/relock logic

As of SERVER-71191, there are two places index builds where we follow the same logic of acquiring the RSTL, releasing it, taking a strong collection lock, and then retrying to acquire RSTL with a timeout. In particular, we do this during index build setup and again for index build commit. This sequence is necessary to avoid deadlocks with prepared transaction and replication state transitions. We should try to consolidate this logic.