Validate keys when a raw connection is checked out with #with

[KJTsanaktsidis]

If a raw connection is obtained via #with, this commit causes that connection to in fact be wrapped in a delegator. The delegator will enforce that any keys used with the raw connection belong to the slot that the #with call was pinned to.

---

This is a replacement for #314 which takes your feedback into account:

• There is a config option for whether or not slot validation is enabled
• The validation is achieved through proxying, rather than subclassing

I do want to give a bit of insight into why I designed it this way, using SimpleDelegator and dynamically detecting which methods to respond to and such. IMO there are two kinds of simplicity we can have here - a simple implementation, which means the code in here is simple, or a simple interface, which means we present an API which does what people expect and is hard to misuse.

In my opinion, it's more important for the interface to be simple, than for the implementation to be simple. In this case, if we're proxying an object, a "simple, unsurprising interface" is that it has exactly the same methods which the underlying object has. That means not missing any methods which RedisClient has (SimpleDelegator takes care of that for us), and not adding any additional methods which e.g. RedisClient:Multi doesn't normally have (i.e. the check for define_singleton_method if respond_to?(method).

It makes the inside of the PinnedConnectionDelegator implementation a little more complex, but it makes it less surprising to use for callers, which I think is the most important thing.