Support default completion tokens in redis::connection

[anarthal]

The non-templated connection should have asio::deferred as the default completion token for all its operations, so the following works:

redis::connection conn (ctx);
// ...
co_await conn.async_exec(req, res);

[mzimbres]

Hi, I did have default completion tokens but somehow they went missing after a refactoring. Will do it when I have the time. Thanks.