Connection: create a connectionState process

In order to check if a connection is open, instead of relying on a ETS table & the connection process, we suggest to delegate this to a new process.

Basically we want:

- Connection handles only the socket
- ConnectionState is used to check connection (without checking the socket), and probably later have stats etc.

_Originally posted by @samuelmanzanera in https://github.com/archethic-foundation/archethic-node/pull/1100#discussion_r1231305227_

Locate the Connection process and the ETS table currently used to determine whether a connection is open. Read how socket handling and connection checks are separated, then define the ConnectionState process boundary. Done means connection checks use ConnectionState without checking the socket, while Connection continues to handle only the socket.

Written by the indexing model from the issue text.