[YSQL] Improve Defaults for Index Backfill Performance

[mrajcevic01]

Jira Link: DB-16055

Description

I recently ran a test with tweaks to our default index backfill performance flags and found it to finish 27% faster:

Flags changed:

backfill_index_timeout_grace_margin_ms: 60000
backfill_index_write_batch_size: 2000

Results:

--without backfill gflag changes
test-database=> create index test on transactions (user_id);
CREATE INDEX
Time: 5114479.282 ms (01:25:14.479)

--with backfill gflag changes (27% improvement)
test-database=> create index test2 on transactions (user_id);
CREATE INDEX
Time: 3735466.539 ms (01:02:15.467)

This table is 170GB large and the index ends up being ~70GB. We should do some further testing to validate these improvements (and cons) before changing the defaults, but seems promising.

Issue Type

kind/enhancement

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