Rename `RedisCommandContext` to `RedisCommand`, improve `RedisCommandHandler` usage semantics, and cleanup documentation.

Motivation:

During proposal review, it was noted that `RedisCommandContext` was a bit misleading, and the hidden reference semantics worrisome.

In addition, several of parts of the documentation around `RedisCommandHandler` were weak or also misleading.

Modifications:

- Rename `RedisCommandContext` to just `RedisCommand`
- Update documentation to be more explicit about the module who owns the types being referenced
- Update documentation to call out explicit usage semantics and behavior
- Change `RedisCommandHandler` to close the socket connection on error thrown
- Rename the "base" Redis Channel Handlers to be more explicitly named

Result:

Users should have clearer documentation on what happens when using `RedisCommandHandler` and `RedisCommand` without hidden semantics.

This contributes to issue #47.