MXS-1777 Turn server weights to their inverses, and make them doubles.

The math becomes simpler when the weight is inverted, i.e. a simple multiplication
to get the (inverse) score. Inverse weights are normalized to the range [0..1] where a lower
number is a higher weight,
The enum select_criteria_t is used to provide a std::function that takes the backends
as vector  (rather than the prior pairwise compares) and returns the best backend.

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -307,7 +307,7 @@ void RWSplit::diagnostics(DCB *dcb)
         for (SERVER_REF *ref = service()->dbref; ref; ref = ref->next)
         {
             dcb_printf(dcb, "\t\t%-20s %3.1f%%     %-6d  %-6d  %d\n",
-                       ref->server->name, (float)ref->weight / 10,
+                       ref->server->name, (1.0 - ref->inv_weight) * 100,
                        ref->server->stats.n_current, ref->connections,
                        ref->server->stats.n_current_ops);
         }