GIN pending list clean up exposure to SQL

This is needed because there needs to be a way to offload this duty from the user backends, and the only other way to intentionaly clean up the list is by vacuum (and the rest of a vacuum can take days to run on a large table).  Autoanalyze will also do it, but it hard to arrange for those to occur at need, and unless you drop default_statistics_target very low they can also take a long time.  And if you do lower the target, it screws up your statistics, of course.

I've currently crammed it into pageinspect, simply because that is where I found the existing code which I used as an exemplar for writing this code.

But where does this belong?  Core?  Its own separate extension?

Cheers,

Jeff