`try-runtime`::`follow-chain` - keep connection (#12167)

* Refactor RPC module

* Add flag to `follow-chain`

* Multithreading remark

* fmt

* O_O

* unused import

* cmon

* accidental removal reverted

* remove RpcHeaderProvider

* mut refs

* fmt

* no mutability

* now?

* now?

* arc mutex

* async mutex

* async mutex

* uhm

* connect in constructor

* remove dep

* old import

* another take

* trigger polkadot pipeline

* trigger pipeline