l string protocol.

    This is a mixin for string protocols (L{Int32StringReceiver},
    L{NetstringReceiver}) which translates L{stringReceived} into a callback
    (prefixed with C{'proto_'}) depending on state.

    The state C{'done'} is special; if a C{proto_*} method returns it, the
    connection will be closed immediately.

    @ivar state: Current state of the protocol. Defaults to C{'init'}.
    @type state: C{str}
    Z