Simplify `RedisCommandExecutor` and `RedisConnection`

Nathan Harrisrequested to merge
redis-client into master

Motivation:

RedisCommandExecutor was a complex and "wordy" name that was not 100% clear as to how it relates to other types.

RedisConnection also has not had a strong use case shown for it to exists as a separate protocol - using up a great name for the "out of the box" implementation.

Result:

RedisClient instead of RedisCommandExecutor is more clear as to what it is, in Redis terminology, a communication client.

RedisConnection as a concrete class provides an identifiable basic block for making connections to Redis.

RedisConnection also saw some fixes to close() while having some names and comment blocks tweaked for updated naming.

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