Added a missing lock acquisition in /__processes__.

Review: https://reviews.apache.org/r/17519
Updated the semantics of Clock::resume.

Now Clock::resume results in the resumption of time from the state
of Time when resume is called, rather than re-winding time back to
real time.

This requires an update to Time::create to take into account the
advanced state of time.

Review: https://reviews.apache.org/r/17480
Fixed the use of a raw unix time in the Slave.

This updates the slave to use Time::create to ensure the modification
time matches the current time frame in libprocess.

Review: https://reviews.apache.org/r/17481
Added a child Reaper utility in libprocess.

Review: https://reviews.apache.org/r/17304
Update the slave to use the libprocess Reaper.

Review: https://reviews.apache.org/r/17305
Added an asynchronous subprocess utility.

Review: https://reviews.apache.org/r/17306