Update slave to use 50% of the disk space for machines with
fewer than 10GB of disk space.

Review: https://reviews.apache.org/r/19427
Fixed the format of DateUtils::currentDate().

Review: https://reviews.apache.org/r/19538
Changed review bot trigger to include dependency changes.

Review: https://reviews.apache.org/r/19480
Refactored the Registrar to push the operations to the callers.

Review: https://reviews.apache.org/r/19372
Changed tabs to spaces in master.cpp.
Fixed a bug to properly re-register an authenticating framework.

Review: https://reviews.apache.org/r/19542