Separate ObjectClient interface

[jtlisi]

This is a followup to a recent discussion that occurred in #2049.

Between S3/GCS/Azure/BoltDB, there is a lot of duplicated code between our chunk object stores. Specifically the GetChunks and PutChunks functions. After #2049, each of these backends supports the follow functions:

type ObjectClient interface {
  PutObject(ctx context.Context, objectKey string, object io.ReadSeeker) 
  GetObject(ctx context.Context, objectKey string) (io.ReadCloser, error)
  List(ctx context.Context, prefix string) ([]StorageObject, error)
}

For each of the Object backend stores, the current ObjectClient interface is implemented using the above functions.

As mentioned in the PR, I propose the following refactor:

• Rename ObjectClient to Client without changing any functionality. Client is chosen as opposed to ChunkClient since the package name is chunk
• Create a new ObjectClient interface that contains the the PutObject, GetObject and List functions
• Create a generic implementation of the Client that would be built on top of the ObjectClient. This would allow a single Client to abstract differences between S3/GCS/AzureBS/BoltDB. It would also allow the ObjectClient to be used to store things other than chunks. DeletePlans or rule configs for example.