Change to parallel uploading for Azure blob storage

[wwwjn]

Reasons for making this change

Related issues

#4286

Screenshots

Checklist

• I've added a screenshot of the changes, if this is a frontend change
• I've added and/or updated tests, if this is a backend change
• I've run the pre-commit.sh script
• I've updated docs, if needed

[wwwjn]

Test result:
TLDR: After using parallel, we can reduce the waiting time when some block is being committed.

In PR #4275, we minimize the size of MIN_WRITE_SIZE and MAX_WRITE_SIZE. This means when uploading a file, we need more put() operations and more commit() operations, which slow down the uploading process. So we need to parallel uploading process to speed up.

Measure the actual upload time of a file content:

1. without parallel: 17625s

2. with parallel: 2690s