Hds

Initial changes for WAL
Wrote tests for
- WAL reading and writing (number of tuples read is same as number of tuples wrote).
- WAL rolling based on size.
- WAL recovery.

Pending changes are
- Integration with HDSStore.
- DefaultWalManager is not required, its functionality to be merged with BucketManager.
Won't need type parameter if we keep serialization out from here.