pgsql: Reset the binary heap in MergeAppend rescans.

Reset the binary heap in MergeAppend rescans.

Failing to do so can cause queries to return wrong data, error out or crash.
This requires adding a new binaryheap_reset() method to binaryheap.c,
but that probably should have been there anyway.

Per bug #8410 from Terje Elde.  Diagnosis and patch by Andres Freund.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/16e8e36cebba8909ec01ca259f32e8dd40090657

Modified Files
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src/backend/executor/nodeMergeAppend.c |    1 +
src/backend/lib/binaryheap.c           |   20 +++++++++++++++++---
src/include/lib/binaryheap.h           |    1 +
3 files changed, 19 insertions(+), 3 deletions(-)