Add sync test and refactor InParallel

[tejal29]

Some work from #2091

• Added a test to make sure InSequence builds are run in sequence.
• Cleaned up parallel.go by adding functions.
• Used sync.Map to collect parallel build results which are safe to use since as per the docs
  The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.
  In our case, one go subroutine adds exactly one key and they are always disjoint.
• Added Individual unit tests.
• Test cases cover code branches and not just errors.

[codecov-io]

Codecov Report

Merging #2118 into master will increase coverage by 0.28%.
The diff coverage is 92.72%.

@@            Coverage Diff             @@
##           master    #2118      +/-   ##
==========================================
+ Coverage   57.31%   57.59%   +0.28%     
==========================================
  Files         186      187       +1     
  Lines        7871     7950      +79     
==========================================
+ Hits         4511     4579      +68     
- Misses       2948     2958      +10     
- Partials      412      413       +1

Impacted Files	Coverage Δ
pkg/skaffold/build/parallel.go	93.44% <92.72%> (+3.64%)	⬆️
cmd/skaffold/app/cmd/build.go	64% <0%> (-4.43%)	⬇️
cmd/skaffold/app/cmd/delete.go	61.53% <0%> (-2.1%)	⬇️
pkg/skaffold/config/options.go	92.68% <0%> (-0.18%)	⬇️
cmd/skaffold/app/cmd/flags.go	92.59% <0%> (ø)
cmd/skaffold/app/cmd/cmd.go	74.57% <0%> (+1.24%)	⬆️

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