Check for artifacts in cache in parallel

[priyawadhwa]

This PR checks for artifacts in the cache in parallel, continues work on #1740

On my desktop, with all cache hits, I noticed these speedups after adding this parallelization:

hipster shop microservices example (11 images)

with this PR:

Cache check complete in 800.688356ms
Cache check complete in 785.371388ms
Cache check complete in 820.054476ms

currently at HEAD:

Cache check complete in 5.104418923s
Cache check complete in 5.149867178s
Cache check complete in 5.250693574s

skaffold/examples/microservices (2 images)

with this PR:

Cache check complete in 507.760169ms
Cache check complete in 533.770901ms
Cache check complete in 497.994918ms

currently at HEAD:
Cache check complete in 1.004895467s
Cache check complete in 1.552249704s
Cache check complete in 1.12099431s

[codecov-io]

Codecov Report

Merging #1799 into master will increase coverage by 0.04%.
The diff coverage is 82.6%.

@@            Coverage Diff             @@
##           master    #1799      +/-   ##
==========================================
+ Coverage   45.48%   45.53%   +0.04%     
==========================================
  Files         142      142              
  Lines        6617     6626       +9     
==========================================
+ Hits         3010     3017       +7     
- Misses       3306     3307       +1     
- Partials      301      302       +1

Impacted Files	Coverage Δ
pkg/skaffold/build/cache/retrieve.go	86.33% <82.6%> (-0.6%)	⬇️

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[balopat]

this is awesome!