Support passing a header object to MsgBase.get_msg_body() and

MsgBase.get_msg_tail() which will optimize the get to avoid re-reading
the headers.  When this is specified, the message number or id is not
required (but specifying it won't hurt anything and will allow backward
compatibility).

I'm not sure I need to lock the header here, but I think I might (if nothing
else, to ensure the message still exists).