Call to a potentially blocking function in aio::connect_simple

[thalesfragoso]

connect_simple (which is async) calls get_socket_addrs:
https://github.com/mitsuhiko/redis-rs/blob/b8eb47303c5728e943b914cd10dc35d1fceee05c/src/aio.rs#L459

Which will end up calling the libstd implementation of ToSocketAddrs for str, this will block if the str is a hostname:
https://github.com/mitsuhiko/redis-rs/blob/b8eb47303c5728e943b914cd10dc35d1fceee05c/src/aio.rs#L496

The usual way to solve this is to wrap the call to to_socket_addrs in a "spawn_blocking", both tokio and async-std have utility functions for that. I guess we could either add spawn_blocking to the RedisRuntime trait, or a more specific method, like get_socket_addrs or lookup_host. I think I would prefer the most general spawn_blocking, since it can be useful for other stuff.

I can provide a PR once there's a minimal consensus on the proper solution.

[tasn]

Related to #379

[doyshinda]

This was fixed in #606