44429: sql: propagate row-level locking modes through execbuilder to row.Fetcher r=nvanbenschoten a=nvanbenschoten

Relates to #40205.

This commit is a follow-up to #44015. It propagates the row-level locking modes
from the optbuilder all the way down to the row.Fetcher. This requires passing
the locking information through the execbuilder and then through DistSQL. The
commit leaves off at the point where `roachpb.ScanRequest`s are constructed.

I couldn't find a good way to test this. It's possible I missed something and
would love any tips, but I think we may need to wait until the plumbing into
the KV API is complete before performing end-to-end tests.

The next step will be to hook the locking modes into the KV API.

Release note: None

44446: roachtest: don't use 20.1 nodelocal on older versions r=dt a=dt

The ability to target the nodelocal directory *of a particular node* was added in 20.1,
so when roachtesting earlier versions we still need to use the old google cloud storage bucket.

Release note: none.

44461: kv: small readability improvement to the heartbeat r=andreimatei a=andreimatei

Rearrange protection against a race to make it more obvious. A casual
reading suggested to me that this protection was missing.

Release note: None

44463: kv: simplify retriable error handling r=andreimatei a=andreimatei

The contract of handleRetryableErrLocked() was weird for no discernible
reason.

Release note: None

Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
Co-authored-by: David Taylor <tinystatemachine@gmail.com>
Co-authored-by: Andrei Matei <andrei@cockroachlabs.com>