Review Request: Added 'unschedule' operation to the GarbageCollector, changed schedule to discard the future when unscheduled/rescheduled, added GarbageCollector unit tests.

[Ben Mahler]

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https://reviews.apache.org/r/10028/
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Review request for mesos, Benjamin Hindman and Vinod Kone.


Description
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See above. This was needed in order to write a fix for MESOS-396.


This addresses bug MESOS-396.
    https://issues.apache.org/jira/browse/MESOS-396


Diffs
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  src/slave/gc.hpp c5cceb13b907c222fc177f1bdeeb838ba8efcc4e 
  src/slave/gc.cpp 92ea79c126f0852a9aeaacd51e7261962f56b8d1 
  src/tests/gc_tests.cpp a9a7a439702ab87d5384c2d1b8afc0b17dab4e62 

Diff: https://reviews.apache.org/r/10028/diff/


Testing
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Added missing and new unit tests.

Also ran the tests with 300 iterations.


Thanks,

Ben Mahler