Simplified-LTS : Convert synchronous ChunkStorage and ChunkMetadataStore API into async.

[sachin-j-joshi]

Problem description
To enable incremental approach and to avoid complexity a few APIs were implemented as synchronous APIs in initial implementation of SLTS.
This is a tracking issue for planned work to convert SLTS APIs to async version

Problem location

• ChunkStorage interface and implementations
• ChunkMetadataStore interface implementations

Suggestions for an improvement
Redefine the interface methods to return CompletableFuture<> . Refactor code as needed.