Algorithm used to wait to split into parent can be unfair

There are cases when threads can wait for a long time for a chance to split into their parent. This is possible in a workload that:

• Has a lot of cache pressure (i.e: relatively small cache and a large number of threads).
• Generates in-memory internal pages with lots of entries.

I have seen cases where a thread has waited for over 3 seconds in __split_parent before getting in to do an update.