release-21.1: kv: properly handle intent pushes to synthetic timestamps

[nvanbenschoten]

Backport 1/1 commits from #63800.

/cc @cockroachdb/release

---

This commit reworks the timestamp that pushers use when pushing intents
to synthetic timestamps. Usually, as of #59086, a pusher limits the
timestamp that it pushes conflicting intents to using the local HLC
clock. This is based on the assumption that it will be able to ignore
any intent above the local HLC clock's reading using observed timestamps
and a local uncertainty limit.

However, this argument only holds if we expect to be able to use a local
uncertainty limit when we return to read the pushed intent. Notably,
local uncertainty limits can not be used to ignore intents with
synthetic timestamps that would otherwise be in a reader's uncertainty
interval. This is because observed timestamps do not apply to
intents/values with synthetic timestamps. So if we know that we will be
pushing an intent to a synthetic timestamp, we don't limit the value to
a clock reading from the local clock.

This came out of some exploration of kvnemesis. I don't think this was
actually causing an issue, but it seemed like a potential source of an
infinite push + conflict loop.

Release note (bug fix): Read-write contention on GLOBAL tables no longer
has a potential to thrash without making progress.

[cockroach-teamcity]

This change is 

[andreimatei]

Reviewable status: complete! 1 of 0 LGTMs obtained