Moved `PluginsManager` and `PluginsManagerType` from `servers::plugin::manager` to the `servers::plugin` module.
Added some comments in code.
Implemented function `into()` to the `PluginsManager` struct.
- Better Client struct (TCP and WebSocket in one type)
- WebSocket is now not a proxy to tcp
- Use async-std instead a tokio
- Use Arc and Client type now have a Clone derive
- Add global CLIENTS list