v2.1: add fanout to tpu-client-next (backport of #3478)

[mergify]

Problem

Crate tpu-client-next doesn't have fan out so far.

Summary of Changes

Add this feature.

---

This is an automatic backport of pull request #3478 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2a618b5 has failed:

On branch mergify/bp/v2.1/pr-3478
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit 2a618b5e01.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tpu-client-next/src/connection_worker.rs
	modified:   tpu-client-next/src/connection_workers_scheduler.rs
	modified:   tpu-client-next/src/leader_updater.rs
	modified:   tpu-client-next/src/send_transaction_stats.rs
	modified:   tpu-client-next/src/workers_cache.rs
	modified:   tpu-client-next/tests/connection_workers_scheduler_test.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Cargo.toml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[alessandrod]

I want to backport this because tpu-client-next is not used by anything yet (0 risk), but we're planning to make it opt-in in send-transaction-service (coming PRs). The existing client (before -next) creates stream concurrency and fragmentation. Both the agave and firedancer QUIC implementations now penalize concurrency, so we want to have a way to avoid it.

[bw-solana]

This is probably a special case because it is all confined to the tpu-client-next crate that isn't actually used anywhere. But in the future, it might be better to split this up into multiple changes that could be reviewed and approved separately. For example, in addition to enabling fanout, we switch to try sends, atomic stats, looking ahead some number of leaders instead of slots, etc.