Cleaning wsrep-API dependencies from library code

Replace all wsrep_api.h dependencies with similar code
in library implementation.

Also add gcc/g++ 4.4 to travis automation to get better
compiler coverage.

include/wsrep/server_state.hpp

@@ -179,6 +179,13 @@ namespace wsrep
             rm_sync
         };
 
+        /**
+         * Magic string to tell provider to engage into trivial (empty)
+         * state transfer. No data will be passed, but the node shall be
+         * considered joined.
+         */
+        static WSREP_CONSTEXPR_OR_INLINE
+        const char* sst_trivial() { return "trivial"; };
 
         virtual ~server_state();