Optional freeClient callback

Adding in an optional freeClient callback to capture errors from the underlying redis library.

cc @squamos @Raynos 

Based on what I needed to do to make sure the test suite doesn't hang, I feel like a single, automatic retry after a bit would be useful to avoid these sorts of errors (and not require redis-manager users to muck with internals, since running freeClient again won't work on clients that ran into such an error).