Per design in YARN-2928, create backing storage read interface that can be implemented by multiple backing storage implementations.
[Storage Implementation] Implement storage reader interface to fetch raw data from HBase backend
Timeline entity uniqueness
[Data Serving] Set up ATS reader with basic request serving structure and lifecycle
Support for fetching specific configs and metrics based on prefixes
Support complex filters in TimelineReader
[Storage implementation] Create HDFS backing storage implementation for ATS reads