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.
Changed map type in Client struct to Arc<Mutex<HashMap<String, ClientMapValue>>>.
Implemented functions insert_key, get_value and delete_key to the Client type.
Re-export servers::server::ClientMapValue in servers::plugins::prelude.