The P2pConfig struct needs a revamp

[TheQuantumPhysicist]

(Created Aug 18, 2023 by @ImplOfAnImpl)

Not all fields of P2pConfig are actual configuration options:

1. Values like msg_header_count_limit and max_request_blocks_count are a part of the protocol, the only reason they are not constants is that we want to be able to modify them in tests. Perhaps, those fields should be moved to a separate struct.
2. Some other fields, like msg_max_locator_count, are not modified even in tests, so probably they should become constants.