Removed internal logging from state::checkpoint().

Review: https://reviews.apache.org/r/13698
Added a re-registration time for slaves.

Review: https://reviews.apache.org/r/13743
Fixed a bug that caused Master::~Master() to send messages to the
schedulers and slaves.

- In production, Masters exit by either LOG(FATAL) or exit().
- Also fixed tests that incorrectly relied on the messages sent by
  Master::~Master().

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/13746
Updated version to 0.15.0.