Support short lived connections

[mzimbres]

To implement sentinel based failover we need better support for short lived connections. At the moment there is no way to launch async_exec and async_run in parallel and cancel when async_exec completes, see this comment.

Also, short lived connections don't need health-check but disabling them here does not prevent their operations from being started here which should be avoided if possible to improve compilation times.

Moreover, for should lived connections it should be possible to send both the HELLO command and the actual request in a single request object, this is currently not possible because the resp3-handshake can't be deactivated here.

We might be able to address these two points by adding an async_run_exec function that only launches the request and the read and write operations.

[mzimbres]

I think it would be just simpler to avoid using the connection for short-lived connections and instead add a free function

auto async_exec(request const&, response&, config, CompletionToken);

This function would then dynamically allocate a redis_stream.

[anarthal]

Are you envisioning this as a detail function or as a public one?

[mzimbres]

A function in detail, I don't see any other usage for this except for failover with sentinels.

A small correction: given that multiple sentinels might be tried before finding one that is responsive, reusing the redis_stream would result in lower overhead and therefore lower latency, perhaps unmeasurable but still. So, the function signature would then have to be

auto async_exec(redis_stream&, request const&, response&, config, CompletionToken);