refactor: rework the proxy engine cache read/write to add concurrency limits

[crandles]

• Adds a limit to concurrent cache chunk operations from single read/write
  • Previously, there was no limit
• Adds early exit to concurrent cache chunk operations, on first error
  • Previously, the cache operations would wait for all chunks to complete before failing the overall request

[coveralls]

Pull Request Test Coverage Report for Build 20218791324

Details

• 268 of 316 (84.81%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 71.84%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/proxy/engines/cache.go	14	16	87.5%
pkg/proxy/engines/cache_write.go	99	111	89.19%
pkg/proxy/engines/cache_read.go	153	187	81.82%

Files with Coverage Reduction	New Missed Lines	%
pkg/proxy/engines/cache.go	2	92.37%

Totals
Change from base Build 20187279647:	0.03%
Covered Lines:	14789
Relevant Lines:	20586

---

💛 - Coveralls