Bug 1096487 - part 1 - make ipdl message constructors set their priority directly; r=bent

The default constructor for ipdl-generated messages looks like:

    MOZ_IMPLICIT Msg_AsyncMessage() :
        IPC::Message(MSG_ROUTING_NONE, ID, IPC::Message::PRIORITY_NORMAL, COMPRESSION_NONE, "PContent::Msg_AsyncMessage")
    {
    }

IPC::Message::PRIORITY_NORMAL is 1, so any set_priority(1) calls are
just redudant.

Additionally, the priorities for messages are known at code generation
time, so we are always able to provide the right constants to the
message constructor, rather than using set_priority calls later.
Bug 1096487 - part 2 - make ipdl message constructors take their routing id; r=bent

Similarly to part, there's no need to say we don't know the routing id
when constructing a message, and then turn around and set the routing id
immediately afterwards.  Just let the constructor do its work.