Review Request: Added double-checked locking when using the paused clock, made sure that processes clocks advanced when events arrived even if they had no sender, did some minor refactorings for delivering events, and changed some logging levels.

[Benjamin Hindman]

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Review request for mesos and John Sirois.


Summary
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See summary.


Diffs
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  third_party/libprocess/include/process/clock.hpp 9480952 
  third_party/libprocess/src/process.cpp 13bb713 

Diff: https://reviews.apache.org/r/5262/diff


Testing
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make check


Thanks,

Benjamin