Now we have situation when openstack projects like Mistral needs
extra oslo.messaging functionality.
But it is too complicated now to to implement something new and
integrate it with current code because there is a little bit mess.
1) Executor should be responsible for how to run jobs
(but now also has code with server logic)
2) Dispatcher should be responsible for routing message to the
target endpoint for processing (but it also has serialisation, sending replies,
executing some executor's callbacks etc)
3) Server should do all server specific logic, we need to have different
implementation of servers for RPC and notification, not different implementations
of dispatchers
This patch fixes 1-st point
Change-Id: Ib6408f408889bb7b7056722be636a5547b1a780d