ARROW-2595: [Plasma] Use map.find instead of operator[] to avoid producing garbage data

[wumuzi520]

• Problem

  • Using object_get_requests_[object_id] will produce a lot of garbage data in PlasmaStore::return_from_get. During the measurement process, we found that there was a lot of memory growth in this point.

• Solution

  • Use iterator instead of operator []

[pcmoritz]

Thanks, good catch!

[codecov-io]

Codecov Report

Merging #2056 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2056      +/-   ##
==========================================
+ Coverage   87.44%   87.44%   +<.01%     
==========================================
  Files         189      189              
  Lines       29374    29376       +2     
==========================================
+ Hits        25686    25688       +2     
  Misses       3688     3688

Impacted Files	Coverage Δ
cpp/src/plasma/store.cc	88.7% <100%> (+0.05%)	⬆️

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[pcmoritz]

@wumuzi520 Thanks for the contribution! Can you create an account in JIRA so I can assign the issue to you?

[wesm]

@wumuzi520 could you create a JIRA account and let us know your user ID? Thank you