pgsql: Ensure that seqscans check for interrupts at least once per page

Ensure that seqscans check for interrupts at least once per page.

If a seqscan encounters many consecutive pages containing only dead tuples,
it can remain in the loop in heapgettup for a long time, and there was no
CHECK_FOR_INTERRUPTS anywhere in that loop.  This meant there were
real-world situations where a query would be effectively uncancelable for
long stretches.  Add a check placed to occur once per page, which should be
enough to provide reasonable response time without adding any measurable
overhead.

Report and patch by Merlin Moncure (though I tweaked it a bit).
Back-patch to all supported branches.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/276bea211f08a3b0d6d5753b654322c8efb781d1

Modified Files
--------------
src/backend/access/heap/heapam.c |    7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)