Use threads for uploads

[JanStevens]

Original implementation from #284

Updated based on the comments, for us we saw our CI going from 12min to 3min

By default it's disabled (and using the old logic)

[coveralls]

Pull Request Test Coverage Report for Build 381

• 13 of 14 (92.86%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 64.726%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/asset_sync/engine.rb	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
lib/asset_sync/engine.rb	1	0.0%

Totals
Change from base Build 376:	0.5%
Covered Lines:	8874
Relevant Lines:	13710

---

💛 - Coveralls

[PikachuEXE]

Thanks for PR
Can you please update it so that it looks like other boolean config like include_manifest?
YML support and stuff
Also need test case

[JanStevens]

Ping small friendly reminder if you got the time :)

[PikachuEXE]

Oh ya new commits won't trigger a notification on GitHub usually
Anyway will take a look tomorrow or later this week

[PikachuEXE]

And released in 2.9.0