Reset the binary heap in MergeAppend rescans.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 16e8e36cebba8909ec01ca259f32e8dd40090657
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2013-08-30T23:15:32Z
Releases: 9.3.0
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.

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