Improve index check speed

[keithf4]

Creating an issue to document some discussions I've had about improving index check speed by using pgstatindex().

empty_pages/deleted_pages account for entire pages that are "bloat"
avg_leaf_density provides a % free value for leaf pages that have tuples in them

The number of "internal_pages" as a function of the index size could also be used to find issues where a reindex would be recommended. Would likely have to be a trended metric and then compare numbers before/after a reindex to see when it would be recommended again in the future.

Provide additional columns returned by pgstatindex in the index stats table. May not use them all in the most common output, but if pgstatindex() is already being run, it can at least provide those stats for other use cases.