Add a PathHandler implementation for Azure blob.

[davides]

Notes

Adds a new PathHandler to support read/write with Azure Blob Storage. Some controls have been put in place so that read/write operations use a known amount of memory when dealing with larger files:

• _open("wb", buffering=<buffer-size>) will buffer up to the requested amount of data in memory before flushing it to the service with the PutBlock operation
• _open("rb", buffering=<buffer-size>) will use the Blob client's chunk iterator to only download a fixed amount of data at a time
• _close() in write-mode will flush any buffered data with one more PutBlock, and finalize the blob with PutBlockList

The block-based approach should work for both block blobs and append blobs (see the Azure docs).

Testing

$ python -m tests.test_azure_blob
......
----------------------------------------------------------------------
Ran 6 tests in 1.992s

OK

[sujitoc]

Can you move .gitignore to a separate PR with just that change?

[sujitoc]

Discussed with @davides and we decided the remaining issue(s) can be addressed in a follow up.

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