Investigate/Improve performance of document expiry throughput

Investigate improve the expiry processing so that

• Its throughput is more predictable, so that we can perform reliable sizings
• It better scales to multicore, so that the available CPU can be utilized for expiry handling

Possible areas to investigate (after profiling) are:

• Performing expiration across multiple vBuckets in parallel.
• Looking at the locking here - is the ItemPager slowed when visiting due to having to hold HashTable lock; or is it the locking when we bulk-delete all expired items in a given vBucket?
• Allowing expiration to skip over HashTable locks which are currently locked - although this would mean StoredValues could be skipped from being considered in a given item pager pass.