release-24.1.0-rc: rangefeed: fix sendMetadata deadlock

[blathers-crl]

Backport 2/2 commits from #123519 on behalf of @msbutler.

/cc @cockroachdb/release

---

If the rangefeed client's context is cancelled before a metadata event is sent
on the event channel, the rangefeed could hang forever. This bug was introduced
in #123001.

Fixes #123355
Fixes #123397

Release note: none

---

Release justification: prevents pcr deadlock

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[jbowens]