Add blocking client

[Darksonn]

This PR adds a simple blocking Redis client.

Open questions:

1. Does the client spawn any tasks, e.g. for periodic pings? If so, a different approach is needed.
2. Should this be a separate crate?
3. Should the module be called sync_client instead?

This is introduced for the purpose of being the basis for an article in the tutorial about using async code in synchronous projects.

[carllerche]

A short explanation of why a current_thread runtime is used here would be nice.

[carllerche]

Does the client spawn any tasks, e.g. for periodic pings? If so, a different approach is needed.

It does not, but this probably should be called out in the comments. Probably the same place the current_thread discussion happens.

Should this be a separate crate?

No, this is fine.

Should the module be called sync_client instead?

Your choice :) Both seem fine to me. In tokio, we use blocking to mean block the thread when waiting, so it seems fine to stick w/ that.