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.
Exposed recovery errors that are encountered in non-strict
recovery mode.

Review: https://reviews.apache.org/r/13789
Added "SlaveRecoveryTest.MasterFailover" test.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/13747
Added SlaveRecoveryTest.MultipleFrameworks test.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/13786
Added a slave recovery test for the case of multiple slaves.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/13753
Added a recovery timeout for executor driver self-termination.

Review: https://reviews.apache.org/r/13791