Support application metadata through ZMQ_METADATA

Lets the application set per-connection metadata.
Metadata is specified as "X-key:value" and set using zmq_setsockopt, eg:
zmq_setsockopt (s, ZMQ_METADATA, "X-key:value", 11);

The peer can then obtain the metadata from a received message:
char *data = zmq_msg_gets(msg, "X-key");

src/options.hpp

@@ -33,6 +33,7 @@
 #include <string>
 #include <vector>
 #include <set>
+#include <map>
 
 #include "atomic_ptr.hpp"
 #include "stddef.h"
@@ -258,6 +259,9 @@ struct options_t
 
     // Use zero copy strategy for storing message content when decoding.
     bool zero_copy;
+
+    // Application metadata
+    std::map<std::string, std::string> app_metadata;
 };
 
 int do_getsockopt (void *const optval_,