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/io_thread.cpp

@@ -26,9 +26,8 @@
 #include "err.hpp"
 #include "ctx.hpp"
 
-zmq::io_thread_t::io_thread_t (ctx_t *ctx_,
-      uint32_t thread_slot_) :
-    object_t (ctx_, thread_slot_)
+zmq::io_thread_t::io_thread_t (ctx_t *ctx_, uint32_t slot_) :
+    object_t (ctx_, slot_)
 {
     poller = new (std::nothrow) poller_t;
     zmq_assert (poller);