Add redis-backed storage class.

[phillbaker]

• Allows keys to be namespaced
• On failure of redis, uses a fallback state (defaults to closed, ie allowing calls through)
• Separates the tests into mixins to be tested against different storage backends (sorry the diff is hard to read).

Questions:

• What do you think about logging the connection failures (so they're not totally invisible)?
• The current code should handle some concurrency - e.g. incrementing the counter at the same time or setting the opened_at timestamp at the same time, in cases where locking is done in local memory, what are your thoughts on translating that to Redis?

Remaining work:

• [x] finish tests covering new functionality of Redis backing
• [x] add concurrency tests for redis, similar to CircuitBreakerThreadsTestCase

[danielfm]

What do you think about logging the connection failures (so they're not totally invisible)?

That would be nice, yes.

The current code should handle some concurrency - e.g. incrementing the counter at the same time or setting the opened_at timestamp at the same time, in cases where locking is done in local memory, what are your thoughts on translating that to Redis?

I think it would be nice to leverage Redis' native features to handle concurrency, as it seems you're doing already. 👍

[phillbaker]

Added logging as well - @danielfm when you get a chance another review would be great.

[phillbaker]

@danielfm any thoughts on this?