pgsql: Do a conditional SPI_push/SPI_pop when replanning a query in

Log Message:
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Do a conditional SPI_push/SPI_pop when replanning a query in
RevalidateCachedPlan. This is to avoid a "SPI_ERROR_CONNECT" failure when
the planner calls a SPI-using function and we are already inside one.
The alternative fix is to expect callers of RevalidateCachedPlan to do this,
which seems likely to result in additional hard-to-detect bugs of omission.
Per reports from Frank van Vugt and Marek Lewczuk.

Back-patch to 8.3. It's much harder to trigger the bug in 8.3, due to a
smaller set of cases in which plans can be invalidated, but it could happen.
(I think perhaps only a SI reset event could make 8.3 fail here, but that's
certainly within the realm of possibility.)

Tags:
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REL8_4_STABLE

Modified Files:
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pgsql/src/backend/utils/cache:
plancache.c (r1.27 -> r1.27.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/plancache.c?r1=1.27&r2=1.27.2.1)