feat: Use multiple threading for chunked upload

[coveralls]

Pull Request Test Coverage Report for Build 4322289600

• 42 of 43 (97.67%) changed or added relevant lines in 2 files are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 93.527%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
boxsdk/util/chunked_uploader.py	41	42	97.62%
<!--	Total:	42	43	97.67%	-->

Files with Coverage Reduction	New Missed Lines	%
boxsdk/object/retention_policy.py	2	78.13%
boxsdk/client/client.py	11	86.14%
<!--	Total:	13		-->

Totals
Change from base Build 4113948266:	0.05%
Covered Lines:	3439
Relevant Lines:	3677

---

💛 - Coveralls

[CLAassistant]

All committers have signed the CLA.