pgsql: Prevent query-lifespan memory leakage of SP-GiST traversalvalue

Prevent query-lifespan memory leakage of SP-GiST traversal values.

The original coding of the SP-GiST scan traversalValue feature (commit
ccd6eb49a) arranged for traversal values to be stored in the query's main
executor context.  That's fine if there's only one index scan per query,
but if there are many, we have a memory leak as successive scans create
new traversal values.  Fix it by creating a separate memory context for
traversal values, which we can reset during spgrescan().  Back-patch
to 9.6 where this code was introduced.

In principle, adding the traversalCxt field to SpGistScanOpaqueData
creates an ABI break in the back branches.  But I (tgl) have little
sympathy for extensions including spgist_private.h, so I'm not very
worried about that.  Alternatively we could stick the new field at the
end of the struct in back branches, but that has its own downsides.

Anton Dignös, reviewed by Alexander Kuzmenkov

Discussion: https://postgr.es/m/CALNdv1jb6y2Te-m8xHLxLX12RsBmZJ1f4hESX7J0HjgyOhA9eA@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/57ef2da434e235f6295bf49a065fce3da398fd8b

Modified Files
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src/backend/access/spgist/spgscan.c | 9 ++++++++-
src/include/access/spgist_private.h | 1 +
2 files changed, 9 insertions(+), 1 deletion(-)