WIP: Socket migration between threads, new zmq_close() semantics

Sockets may now be migrated between OS threads; sockets may not be used by
more than one thread at any time. To migrate a socket to another thread the
caller must ensure that a full memory barrier is called before using the
socket from the target thread.

The new zmq_close() semantics implement the behaviour discussed at:

http://lists.zeromq.org/pipermail/zeromq-dev/2010-July/004244.html

Specifically, zmq_close() is now deterministic and while it still returns
immediately, it does not discard any data that may still be queued for
sending. Further, zmq_term() will now block until all outstanding data has
been sent.

TODO: Many bugs have been introduced, needs testing. Further, SO_LINGER or
an equivalent mechanism (possibly a configurable timeout to zmq_term())
needs to be implemented.

src/thread.hpp

@@ -54,15 +54,6 @@ namespace zmq
 
         //  Waits for thread termination.
         void stop ();
-
-#ifdef ZMQ_HAVE_WINDOWS
-        typedef DWORD id_t;
-#else
-        typedef pthread_t id_t;
-#endif
-
-        static id_t id ();
-        static bool equal (id_t id1_, id_t id2_);
         
     private: