This adds basic coverage to IO threads by running the cluster and few selected Redis test suite tests with the IO threads enabled.
Also provides some necessary additional improvements to the test suite:
* Add --config to sentinel/cluster tests for arbitrary configuration.
* Fix --tags whitelisting which was broken.
* Add a `network` tag to some tests that are more network intensive. This is work in progress and more tests should be properly tagged in the future.
* Introduce a connection abstraction layer for all socket operations and
integrate it across the code base.
* Provide an optional TLS connections implementation based on OpenSSL.
* Pull a newer version of hiredis with TLS support.
* Tests, redis-cli updates for TLS support.
