pgsql: Cache the results of ri_FetchConstraintInfo in a backend-local c

Cache the results of ri_FetchConstraintInfo in a backend-local cache.

Extracting data from pg_constraint turned out to take as much as 10% of the
runtime in a bulk-update case where the foreign key column wasn't changing,
because we did it over again for each tuple. Fix that by maintaining a
backend-local cache of the results. This is really a pretty small patch,
but converting the trigger functions to work with pointers rather than
local struct variables requires a lot of mechanical changes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/45ba424f332a7eaa685a4e23f142a43114c65811

Modified Files
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src/backend/utils/adt/ri_triggers.c | 548 ++++++++++++++++++++---------------
1 files changed, 319 insertions(+), 229 deletions(-)