Optimize AzureBlobStore#delete method

[dadoonet]

When we have to remove a lot of files, method AzureBlobStore#delete is called for each file and generates a Azure API call.

Would be nice to see if we can batch somehow those calls instead.

[dadoonet]

@imotov As far as I understand the core code, I don't really think that it's doable. We need to process file deletion one by one, right?

[imotov]

@dadoonet actually when we delete a lot of files, we have a list of blobs that should be deleted and we simply iterate through the list. We can easily modify the logic to pass entire list to the store.

[dadoonet]

This issue was moved to elastic/elasticsearch#12697