Add batch get, update, delete methods to Storage and Blob

[mziccard]

This PR follows the discussion in #219.
It adds get, update and delete methods that simplify doing batch gets/updates/deletes. Methods are added to both Storage and Blob.
Both unit and integration tests are added.

[aozarov]

Do you think we should also add similar methods to Bucket (e.g. for Bucket.update)?

Now, I noticed that we have Bucket.getAll(String...) and I wonder if signature should be changed to Bucket.get(String, String...)

[mziccard]

The methods in Blob have more sense as they provide a nicer interface to a useful mechanism (batch requests) to enforce its usage. Adding the methods to Bucket is only syntactic sugar (and I wonder even how much it could be used). I don't know, maybe we should add them anyway.

Yes we should probably change Bucket.getAll(String...) to Bucket.get(String, String...).

Should we do all of these in this PR?

[aozarov]

Technically we could also add bucket modification/get to the batch but I agree a need for such feature is low. Lets leave it for now (can be added later if requested).

Doing Bucket.get(String, String...) in this PR seems fine to me.

[aozarov]

I like it now (with the service-level batching and consistent, clean vararg) much better.

I commented about 2 issues regarding exceptions

I actually now re-thinking about the BlobId and wonder if it is actually worth adding it (and use it when appropriate instead of BlobInfo, e.g. in delete[s], get[s],...) provided we also provide the convenient methods (such as get(bucket, name), BlobInfo.builder(bucket, name),...) .
What do you think (in any case, clearly not for this PR)?

[mziccard]

I fixed last isses.

As I said in #62 I think that BlobId will not provide major benefits to users. However, it might help to have more uniform APIs. I'll work on adding it tomorrow, I'll take care of keeping al convenience methods as:

• storage.get(bucket, name)
• BlobInfo.builder(bucket, name) (while dropping BlobInfo.of(bucket, name) in favor of BlobId.of(bucket, name)
• storage.delete(bucket, name)
• storage.reader(bucket, name)

I would also add an id() getter to BlobInfo without delete name() and bucket() so that to avoid writing info.id().name().

[aozarov]

Great, PR looks good and I think we are on the same page regarding #62.