refactor(source): extract concurrency checks and rpc fetches into separate Fetcher struct

[kskalski]

Motivation

Right now semaphore and rate-limiting code is spread and duplicated all over the code-base, some calls to rpc methods are actually missing them.

Solution

Create a wrapper for Provider that centralized all rpc accesses and makes sure concurrency controls are applied to all of them.
In future this might also allow easy introduction of re-tries in presence of network or transient target endpoint errors.

It's probably possible to make Fetcher implement Middleware trait and maybe reduce a bit some of the function signature changes, but at the moment this seemed easier and ensures all used rpc methods properly follows concurrency checks without overriding all middleware functions.

PR Checklist

• [ adjusted existing test ] Added Tests
• [ x ] Added Documentation
• [ none ] Breaking changes

[sslivkoff]

great solution. this cleans up a ton of code