pgsql: Use caller's memory context for radix tree iteration state

Use caller's memory context for radix tree iteration state

Typically only one iterator is present at any time, so it's overkill
to devote an entire context for this. Get rid of it and use the
caller's context.

This is tidy-up work, so no backpatch in this form. However, a
hypothetical extension to v17 that tried to start iteration from
an attaching backend would result in a crash, so that'll be fixed
separately in a way that doesn't change behavior in core.

Patch by me, reported and reviewed by Masahiko Sawada

Discussion: https://postgr.es/m/CAD21AoBB2U47V=F+wQRB1bERov_of5=BOZGaybjaV8FLQyqG3Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/960013f2a1f9e51aad85c91aceeb442508be6032

Modified Files
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src/include/lib/radixtree.h | 15 +++------------
1 file changed, 3 insertions(+), 12 deletions(-)