Reorder log slot release so joins don't wait on IO

We have seen cases on insert heavy workloads where there may be a lot of slot join races. Looking at the code in log_slot_switch_internal the switching thread is calling wt_log_release where we write the buffer contents to the file system on the slot that is still in log->active_slot. Investigate whether we can set up a new slot as the active slot first, and then write the moved-aside slot.