Feature/safe disconnect

Following my previous attempts to increase the resilience of the Redis driver to the Redis server being down, the following is a patch to ensure that we're not going to throw unnecessary exceptions if we weren't able to get connected in the first place.

This patch:

* Fixes `RedisTest::getOptions` which wasn't returning config in the expected format and was subsequently ignored;
* Introduces `RedisTest::getInvalidOptions` which return config pointing to no open port, guaranteed; and
* Ensures that `Stash\Driver\Redis::__destruct` won't throw `\RedisException`s.