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/ctx.hpp

@@ -234,6 +234,9 @@ class ctx_t : public thread_ctx_t
     //  Is IPv6 enabled on this context?
     bool ipv6;
 
+    // Should we use zero copy message decoding in this context?
+    bool zero_copy;
+
     ctx_t (const ctx_t &);
     const ctx_t &operator= (const ctx_t &);