preparations for 0.9 changes

this commit is the first part of a revision of libcppa that removes
the thread-local `self` pointer, `actor_ptr`, and `channel_ptr`
to pave the way for fully type-safe actor programming;
this first step splits the addressing of actors into an `actor`
handle and an `actor_addr`: only the former can be used to send
messages, whereas the latter can only be used to monitor or identify
actors;
next steps will add `typed_actor<>` handles that allow the compiler
to type-check the message passing interface;
the revision is work in progress and this commit does not compile