Problem: enormous memory increase due to zero copy decoding

The zero copy decoding strategy implemented for 4.2.0 can lead to a large
increase of main memory usage in some cases (I have seen one program go up to
40G from 10G after upgrading from 4.1.4). This commit adds a new option to
contexts, called ZMQ_ZERO_COPY_RECV, which allows one to switch to the old
decoding strategy.

src/socket_base.cpp

@@ -206,6 +206,7 @@ zmq::socket_base_t::socket_base_t (ctx_t *parent_,
     options.socket_id = sid_;
     options.ipv6 = (parent_->get (ZMQ_IPV6) != 0);
     options.linger.store (parent_->get (ZMQ_BLOCKY) ? -1 : 0);
+    options.zero_copy = parent_->get (ZMQ_ZERO_COPY_RECV);
 
     if (thread_safe) {
         mailbox = new (std::nothrow) mailbox_safe_t (&sync);