Reader Cleanup Part 2

RFR once github gets their act together and starts showing commits up through 9eaabc8

This cleans up shard state reading and getting columns from cassandra. 
 * Fewer Astyanax internals leak outside of the `.io` package.
 * AstyanaxReader now lets the ShardStateManager worry about UpdateStamp objects. There could potentially be cleaner separation done here, I'll think about dealing with that in later steps of refactoring.
 * Fewer round-trips to Cassandra by batching shard-state reading.
 * Less code duplication in getColumnsFromDB batched vs non-batched.