Migrator Service

[nsivabalan]

If someone is looking to migrate to Ambry after trying out other options, they might have some files on-premise or some other object store. So, definitely they might be looking to migrate those to ambry in a simpler manner. Would be great if we have a migrator service.

Some requirements for a migrator service:

1. Given a root directory, the migrator service should upload all files to ambry and return a map of file names(with paths) to AmbryIds.
2. Define an interface so that the migrator service can poll a service for objects and uploads to ambry. For instance, from hdfs or amazon S3.