Re: All-zero page in GIN index causes assertion failure

I came up with the attached, for GIN and SP-GiST. Instead of WAL-logging the page initialization in SP-GiST vacuum, I changed it so that it simply leaves the page as all-zeroes, and adds it to the FSM. The code that grabs a target page from the FSM handles that, and initializes the page anyway, so that was simpler. This made the SP-GiST is-deleted flag obsolete, it's no longer set, although the code still checks for it for backwards-compatibility. (even that may actually be unnecessary, as a page that's marked as deleted must also be empty, and wherever we check for the deleted-flag, we also check for PageIsEmpty()))