Implementation of Store interface that writes to cloud storage

[lightningrob]

We are starting an effort to build a backup/recovery solution where Ambry blobs are backed up to cloud storage (eg. Azure). To that end, a cloud-facing replication manager will be created, in order to reuse the existing replication logic. The first step in this journey is to add a Store implementation that write data to a cloud backend instead of logs in the file system. The goal is to plug this CloudBlobStore as seamlessly as possible into the existing ReplicationManager/ReplicaThread classes.