Prefixed cmake WITH_ definitions with WSREP_LIB_ to avoid collisions

with superproject definitions.

Avoid using client_service::do_2pc() in before_commit() to
determine if 2pc is actually happening, will use transaction
states to deduce that. client_service::do_2pc() should be deprecated.

Fixed a compiler warning in db_high_priority_service.cpp.

include/wsrep/client_service.hpp

@@ -28,6 +28,10 @@ namespace wsrep
         /**
          * Return true if two pahase commit is required for transaction
          * to commit.
+         *
+         * @todo This interface method should be deprecated, 1pc vs 2pc
+         *       will be deduced from transaction states in
+         *       transaction::before_commit() call.
          */
         virtual bool do_2pc() const = 0;