Delete empty pages during GiST VACUUM.

 Delete empty pages during GiST VACUUM.
 
 To do this, we scan GiST two times. In the first pass we make note of
 empty leaf pages and internal pages. At second pass we scan through
 internal pages, looking for downlinks to the empty pages.
 
 Deleting internal pages is still not supported, like in nbtree, the last
 child of an internal page is never deleted. That means that if you have a
 workload where new keys are always inserted to different area than where
 old keys are removed, the index will still grow without bound. But the rate
 of growth will be an order of magnitude slower than before.
 
 Author: Andrey Borodin
 Discussion: https://www.postgresql.org/message-id/B1E4DF12-6CD3-4706-BDBD-BF3283328F60@yandex-team.ru