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.
This fixes three issues:
1. Using debug SLEEP was impacting the subsequent test, and causing it to pass reliably even though it should have failed. There was exactly 5 seconds of artificial pause (after 1000, wait 3000, wait 1000) between the debug sleep 5 and when we needed to unblock the client in the subsequent test. Now the test properly makes sure the client is unblocked, and the subsequent test is fixed.
2. Minor, the client pause types were using & comparisons instead of ==, since it was previously a flag.
3. Test is faster now that some of the hand wavy time is removed.
* 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.
