Fixed missing inline for cleanup.

Review: https://reviews.apache.org/r/19214
Changed Option::get to return a const reference to reduce copying.

Review: https://reviews.apache.org/r/18383
Option::get reference cleanup in libprocess.

Review: https://reviews.apache.org/r/18384
Option::get reference cleanup in mesos.

Review: https://reviews.apache.org/r/18386