pgsql: Refactor crosstab() to build and return a tuplestore instead of

Log Message:
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Refactor crosstab() to build and return a tuplestore instead of using
value-per-call mode. This should be more efficient in normal usage,
but the real problem with the prior coding was that it returned with
a SPI call still active. That could cause problems if execution was
interleaved with anything else that might use SPI.

Modified Files:
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pgsql/contrib/tablefunc:
tablefunc.c (r1.56 -> r1.57)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/tablefunc/tablefunc.c?r1=1.56&r2=1.57)