Review Request: Fixed the GC of terminated executor directories in the slave. The directories are also unscheduled for gc, when launching another executor of the same id.

[Ben Mahler]

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https://reviews.apache.org/r/10032/
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Review request for mesos, Benjamin Hindman and Vinod Kone.


Description
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This does some restructuring of runTask() in the slave, in order to support the 
asynchronous gc.unschedule() operation of existing executor directories.


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


Diffs
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  src/slave/slave.hpp dadd06850565d07b46cea2f581661fb57a121118 
  src/slave/slave.cpp 912499e4cb34afa9bcc1556fd9536581001933ca 
  src/tests/gc_tests.cpp a9a7a439702ab87d5384c2d1b8afc0b17dab4e62 
  src/tests/utils.hpp 50203579f1ea14ee751c12a373047e9295e4ebb3 

Diff: https://reviews.apache.org/r/10032/diff/


Testing
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Added an integration test for restarted executors.


Thanks,

Ben Mahler