Make Service::run thread safe

[Rui Figueira]

This is so multiple threads can dequeue completed handlers. Pretty much like Asio.
Although by allowing this, it would seriously complicate things for the user, since access to sockets would have to be protected.
For example, to make things easier for the user, some kind of strand implementation would be required, thus increasing czspas codebase.

Something to think about later.