Define undefined id and gtid as const static member variables.

Use default seqno and id constructors in default gtid ctor
instead of calls to static undefined to initialize.

src/id.cpp

@@ -24,6 +24,8 @@
 #include <sstream>
 #include <algorithm>
 
+const wsrep::id wsrep::id::undefined_ = wsrep::id();
+
 wsrep::id::id(const std::string& str)
     :  data_()
 {