Log server version when it changes

Knowing what the server version is can help rule out some
problems. Logging it whenever it changes also helps figure out when
upgrades took place.

server/core/server.cc

@@ -810,6 +810,11 @@ uint64_t SERVER::status_from_string(const char* str)
 
 void Server::set_version(uint64_t version_num, const std::string& version_str)
 {
+    if (version_str != version_string())
+    {
+        MXS_NOTICE("Server '%s' version: %s", name(), version_str.c_str());
+    }
+
     m_info.set(version_num, version_str);
 }