pgsql: Repair incorrect handling of AfterTriggerSharedData.ats_modified

Repair incorrect handling of AfterTriggerSharedData.ats_modifiedcols.

This patch fixes two distinct errors that both ultimately trace
to commit 71d60e2aa, which added the ats_modifiedcols field.

The more severe error is that ats_modifiedcols wasn't accounted for
in afterTriggerAddEvent's scanning loop that looks for a pre-existing
duplicate AfterTriggerSharedData. Thus, a new event could be
incorrectly matched to an AfterTriggerSharedData that has a different
value of ats_modifiedcols, resulting in the wrong tg_updatedcols
bitmap getting passed to the trigger whenever it finally gets fired.
We'd not noticed because (a) few triggers consult tg_updatedcols,
and (b) we had no tests exercising a case where such a trigger was
called as an AFTER trigger. In the test case added by this commit,
contrib/lo's trigger fails to remove a large object when expected
because (without this fix) it thinks the LO OID column hasn't changed.

The other problem was introduced by commit ce5aaea8c, which copied the
modified-columns bitmap into trigger-related storage. It made a copy
for every trigger event, whereas what we really want is to make a new
copy only when we make a new AfterTriggerSharedData entry. (We could
imagine adding extra logic to reduce the number of bitmap copies still
more, but it doesn't look worthwhile at the moment.) In a simple test
of an UPDATE of 10000000 rows with a single AFTER trigger, this thinko
roughly tripled the amount of memory consumed by the pending-triggers
data structures, from 160446744 to 480443440 bytes.

Fixing the first problem requires introducing a bms_equal() call into
afterTriggerAddEvent's scanning loop, which is slightly annoying from
a speed perspective. However, getting rid of the excessive bms_copy()
calls from the second problem balances that out; overall speed of
trigger operations is the same or slightly better, in my tests.

Discussion: https://postgr.es/m/3496294.1737501591@sss.pgh.pa.us
Backpatch-through: 13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ea68ea6320ff84f55cf30dff1af662fc0bf5dafa

Modified Files
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contrib/lo/expected/lo.out | 69 ++++++++++++++++++++++++++++++++++++++++++
contrib/lo/sql/lo.sql | 41 +++++++++++++++++++++++++
src/backend/commands/trigger.c | 18 +++++------
3 files changed, 118 insertions(+), 10 deletions(-)