Pluggable storage classes for the Worker API

[shaneharter]

1. Abstract all of the SHM functionality into an SHM storage class.
2. Create additional storage options (Redis, Memcached, etc) and implement the ability for application developers to choose a specific storage class. Create hueristic that will try to select the optional storage engine based on worker throughput and mean arg/return value size.

The idea here is that we've learned from this first release of the Worker API that shared memory has faults and is, you could say, not the most rock solid PHP API available. And if Redis or Memcached are available on the server, one or both could be a better option than SHM.