pgsql: Don't use static storage for SaveTransactionCharacteristics().

Don't use static storage for SaveTransactionCharacteristics().

This is pretty queasy-making on general principles, and the more so
once you notice that CommitTransactionCommand() is actually stomping
on the values saved by _SPI_commit().  It's okay as long as the
active values didn't change during HoldPinnedPortals(); but that's
a larger assumption than I think we want to make, especially since
the fix is so simple.

Discussion: https://postgr.es/m/1533956.1645731245@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/12d768e70497afc5a57acf73c251316997b5175a

Modified Files
--------------
src/backend/access/transam/xact.c | 32 ++++++++++++++------------------
src/backend/executor/spi.c        | 16 ++++++++--------
src/include/access/xact.h         | 12 ++++++++++--
3 files changed, 32 insertions(+), 28 deletions(-)