introduce vbucket based cache bucket with new configuration and cleanup

Current memcached style buckets are very different than Couchbase buckets without much good reason. Introduction of a new bucket type for caching would help by:

1) Allowing for the new configuration approach to be used by cache buckets which gives us greater scalability and faster response to change in topology
2) Makes fault detection much more reliable and identification of problems more clear to applciations and administrators
3) Allows for significant clean up of huge chunks of code in a future major version by removing memcached buckets