Function watch_spawned 

pub fn watch_spawned(
    provider: &str,
    nick: &str,
    session_id: &str,
    store_root: Option<&str>,
    cwd: Option<&str>,
) -> bool

Starts the mailbox listener of an account this node assigned at spawn: it keeps the account open (sync, push, auto-join of invites by the m4a auto-join policy) and turns each new letter into a wake signal for that session.