Replace STRTARGET macro

The macro was extremely unwieldly to update and made the addition of debug
assertions harder. Rewriting it as an inline function makes this possible.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -344,8 +344,7 @@ bool RWSplitSession::route_single_stmt(GWBUF* querybuf)
         else
         {
             MXS_ERROR("Could not find valid server for target type %s, closing "
-                      "connection.",
-                      STRTARGET(route_target));
+                      "connection.", route_target_to_string(route_target));
         }
     }