Reapply changes from reverted PR 3423: move structures to new modules

[KirillLykov]

Problem

This PR moves structures SendTransactionServiceStatsand TransactionClient (introduced recently) to their own modules: these changes are split into two commits for simplicity of the review. There are no changes in the logic, just moves code to new modules.

Summary of Changes

[KirillLykov]

Not sure about the version here in case of backporting

[t-nelson]

are the changes that are intended to be backported all already in? ideally we do not take a bunch of refactor/reorg/cosmetics through backport. just logic changes

[KirillLykov]

are the changes that are intended to be backported all already in? ideally we do not take a bunch of refactor/reorg/cosmetics through backport. just logic changes

No, in order for the validator to be able to use tpu-client-next, I need to add the following:

1. add new client code to the SendTransactionService (currently it has a client implemented using only ConnectionCache)
2. Plumbing on the rpc crate side to be able to select one of the clients
3. Plumbing on the validatator.rs to select the client and pass it to RPC, RPC will pass to SendTransactionService.

So although these changes do not change much of the logic, they are big code-wise.

Moving data structures to the new place might be postponed and done after (3), bu they are quite risk-free changes.

[KirillLykov]

I think it is reasonable now to retreat on backporting to 2.1 (because there are a lot of changes), so move these structures around

[alexpyattaev]

LGTM

[t-nelson]

nothing after this is intended for backport, right? we aren't backporting reorg/refactor changes

[KirillLykov]

nothing after this is intended for backport, right? we aren't backporting reorg/refactor changes

correct, no backporting