[YUNIKORN-499] race condition in sorting apps

[wilfred-s]

While sorting applications the pending resources of an application are
accessed. This could happen while a different routine is updating the
pending resources when a Ask is modified.
Replace the pending resource object instead of cloning on retrieval as it
is faster.

[codecov]

Codecov Report

Merging #234 (bc649f1) into master (5471d84) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   64.02%   64.02%           
=======================================
  Files          60       60           
  Lines        4984     4984           
=======================================
  Hits         3191     3191           
  Misses       1636     1636           
  Partials      157      157           

Impacted Files	Coverage Δ
pkg/scheduler/objects/application.go	55.30% <100.00%> (ø)

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[yangwwei]

+1, thanks for the quick fix @wilfred-s