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/config.hpp

@@ -27,9 +27,8 @@ namespace zmq
 
     enum 
     {
-        //  Maximal number of OS threads that can own 0MQ sockets
-        //  at the same time.
-        max_app_threads = 512,
+        //  Maximum number of sockets that can be opened at the same time.
+        max_sockets = 512,
 
         //  Number of new messages in message pipe needed to trigger new memory
         //  allocation. Setting this parameter to 256 decreases the impact of