Deprecate the synchrony driver

[byroot]

I'd like to only include the ruby and hiredis drivers in the gem. The synchrony one is hard to debug, and I don't think that many people use it. If there is interest for it I'd rather have people who known em well maintain it as an external gem.

If you've seen the deprecation message and would like to maintain the driver, let me know.

The plan would be to drop it on the next major version (5.0).