pgsql: Sanity-check that a page zeroed by redo routine is marked with W

Sanity-check that a page zeroed by redo routine is marked with WILL_INIT.

There was already a sanity-check in the other direction: if a page was
marked with WILL_INIT, it had to be initialized by the redo routine. It's
not strictly necessary for correctness that a page is marked with WILL_INIT
if it's going to be initialized at redo, but it's a missed optimization if
nothing else.

Fix a few instances of this issue in SP-GiST, where a block in WAL record
was not marked with WILL_INIT, but was in fact always initialized at redo.
We were creating a full-page image of the page unnecessarily in those
cases.

Backpatch to 9.5, where the new WILL_INIT flag was added.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e015c3e51f76a05cc026c8323c51a373172adaa3

Modified Files
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src/backend/access/spgist/spgdoinsert.c | 31 +++++++++++++++++++++++++------
src/backend/access/transam/xlogutils.c | 19 +++++++++++++------
2 files changed, 38 insertions(+), 12 deletions(-)