Start the global RPC server thread
    Returns the RPC server thread

def rpc_start( working_dir, port, subdomain_index=None, thread=True ):
    """
    Start the global RPC server thread
    Returns the RPC server thread
    """
    rpc_srv = BlockstackdRPCServer( working_dir, port, subdomain_index=subdomain_index )
    log.debug("Starting RPC on port {}".format(port))

    if thread:
        rpc_srv.start()

    return rpc_srv