In some corner cases, it is possible for the BRIN index relation to be
extended by brin_getinsertbuffer but the new page not be used
immediately for anything by its callers; when this happens, the page is
initialized and the FSM is updated (by brin_getinsertbuffer) with the
info about that page, but these actions are not WAL-logged.  A later
index insert/update can use the page, but since the page is already
initialized, the initialization itself is not WAL-logged then either.
Replay of this sequence of events causes recovery to fail altogether.

There is a related corner case within brin_getinsertbuffer itself, in
which we extend the relation to put a new index tuple there, but later
find out that we cannot do so, and do not return the buffer; the page
obtained from extension is not even initialized.  The resulting page is
lost forever.

To fix, shuffle the code so that initialization is not the
responsibility of brin_getinsertbuffer anymore, in normal cases;
instead, the initialization is done by its callers (brin_doinsert and
brin_doupdate) once they're certain that the page is going to be used.
When either those functions determine that the new page cannot be used,
before bailing out they initialize the page as an empty regular page,
enter it in FSM and WAL-log all this.  This way, the page is usable for
future index insertions, and WAL replay doesn't find trying to insert
tuples in pages whose initialization didn't make it to the WAL.  The
same strategy is used in brin_getinsertbuffer when it cannot return the
new page.

Additionally, add a new step to vacuuming so that all pages of the index
are scanned; whenever an uninitialized page is found, it is initialized
as empty and WAL-logged.  This closes the hole that the relation is
extended but the system crashes before anything is WAL-logged about it.
We also take this opportunity to update the FSM, in case it has gotten
out of date.

Thanks to Heikki Linnakangas for finding the problem that kicked some
additional analysis of BRIN page assignment code.

Backpatch to 9.5, where BRIN was introduced.

Discussion: https://www.postgresql.org/message-id/20150723204810.GY5596@postgresql.org