Re-think Samza's Window and Join APIs

Currently, Windows can either be Keyed or Non-Keyed , session or tumbling , event time or system-time based windows. In addition to that, they can optionally take in a fold-left function. This combination of various windowing facets results in an API with multiple parameters, and variants. We should make the API more composable.

For windows that need to access the entire list of messages in the window (for compute percentiles for example), we store the list as a single value in RocksDbKVStore. We should build a store abstraction for storing time-series messages.