Reduce bloat due to statics

Avoid disseminating every compile unit including Message.h with TransportNames and
TransportTypes and the associated unordered_map helper methods (e.g.
murmur_hash).

fairmq/Transports.h

@@ -10,6 +10,7 @@
 #define FAIR_MQ_TRANSPORTS_H
 
 #include <fairmq/tools/Strings.h>
+#include <fairmq/TransportEnum.h>
 #include <memory>
 #include <ostream>
 #include <stdexcept>
@@ -18,13 +19,6 @@
 
 namespace fair::mq {
 
-enum class Transport
-{
-    DEFAULT,
-    ZMQ,
-    SHM
-};
-
 struct TransportError : std::runtime_error
 {
     using std::runtime_error::runtime_error;