v2.1: Reapply changes from reverted PR 3423 (backport of #4257)

[mergify]

This PR adds back changes from PR 3423 but this time with less API changes which allows to simplify backport to 2.1.

Problem

In order to be able to use new client code in the SendTransactionService (see #3444), I need to wrap network-related code with the new structure ConnectionCacheClient.

Summary of Changes

To extract network code into a new structure, I had to move some auxiliary code into separate files.
Beside of that, changes are pretty minimal and I deliberately haven't tried to optimize existing code to keep changes minimal. It just does almost exactly the same as the old one.

---

This is an automatic backport of pull request #4257 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4fe5092 has failed:

On branch mergify/bp/v2.1/pr-4257
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit 4fe50924e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Cargo.lock
	modified:   programs/sbf/Cargo.lock
	modified:   send-transaction-service/Cargo.toml
	modified:   send-transaction-service/src/send_transaction_service.rs

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   svm/examples/Cargo.lock

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

[KirillLykov]

Hm, cherry-picking seems to be fine but CI breaks with missing /solana/svm/examples/Cargo.toml

[t-nelson]

the problem is that the corresponding lock file was committed, but not the toml. delete the lock file

[t-nelson]

who's sme for this? @lijunwangs @alessandrod ?

[KirillLykov]

@t-nelson Alessandro has approved.

[KirillLykov]

Just in case, I checked that master with this commit and without this commit doesn't change metrics, along with mean/std for the number of transactions in the blocks for the test-validator. Also the distribution over time is identical:

btw, the gaps are due to the way SendTransactionService is implemented (I haven't changed that because this would be irrelevant to the task).