Allow `const auto &` as an argument for delivery filter in single/multi_sink_binding

[eao197]

The current implementation requires repetition of type name if delivery filter is used with so_5::single_sink_binding_t and so_5::multi_sink_binding_t:

so_5::single_sink_binding_t binding;
binding.bind<MyMsg>(src_mbox, dest_msink, [](const MyMsg & msg) {...});

It would be cool to have a possibility to use const auto& as argument type for delivery filter:

so_5::single_sink_binding_t binding;
binding.bind<MyMsg>(src_mbox, dest_msink, [](const auto & msg) {...});

It may make code more flexible (especially in case of templates).

[eao197]

The fix will be a part of upcoming 5.8.1 release.