Use estimated current slot in tpu client (backport #28997)

[mergify]

This is an automatic backport of pull request #28997 done by Mergify.
Cherry-pick of 07d21d4 has failed:

On branch mergify/bp/v1.14/pr-28997
Your branch is up to date with 'origin/v1.14'.

You are currently cherry-picking commit 07d21d411.
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	deleted by us:   tpu-client/src/nonblocking/tpu_client.rs

no changes added to commit (use "git add" and/or "git commit -a")

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[solana-grimes]

😱 New commits were pushed while the automerge label was present.

[pgarg66]

Backporting the fix to v1.14 as it's causing instabilities in the QUIC client.

[CriesofCarrots]

Just to make the history clear, this looks different from the master PR because we did not backport #28954 to v1.14, correct?

[pgarg66]

Just to make the history clear, this looks different from the master PR because we did not backport #28954 to v1.14, correct?

Yes, that's correct. Also, files have been moved around on master. So I had to manually patch it.