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/options.hpp

@@ -251,6 +251,9 @@ struct options_t
 
     // Use of loopback fastpath.
     bool loopback_fastpath;
+
+    // Use zero copy strategy for storing message content when decoding.
+    bool zero_copy;
 };
 }