Aggregate scheduler times by task class, rather than task

Summary

We currently produce a scheduler histogram (time a task is waiting to be scheduled) based on task names. e.g.

• VBucketMemoryDeletionTask
• ExpiredItemPager
• etc.

Instead it would be more useful to produce the histogram based on the fours thread types, given the scheduler times are not actually a function of a Task itself, but all tasks sharing a scheduler class:

• Reader
• Write
• AuxIO
• NonIO

This will make it much easier to spot if there is contention on a specific thread type.