tableam: VACUUM and ANALYZE support.

tableam: VACUUM and ANALYZE support.

This is a relatively straightforward move of the current
implementation to sit below tableam. As the current analyze sampling
implementation is pretty inherently block based, the tableam analyze
interface is as well. It might make sense to generalize that at some
point, but that seems like a larger project that shouldn't be
undertaken at the same time as the introduction of tableam.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de