Add new API allowing changing dhandle hash bucket size

Follow-on from WT-6421. Even though I was not able to see any difference in checkpoint time and schema lock time held changing the number of hash buckets, there is desire for people to try out various values. This ticket is to add an API to change that setting.

There is a direct tradeoff to changing the hash bucket size, which is an increase in memory consumed. There are many structures that use hash bucket arrays.

I believe this ticket should split the hash buckets into two. One specifically sizing the dhandle hash buckets (used in the connection and each session) and another general sizing for all other hash buckets to help mitigate the memory consumption.