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
Simplified os::hostname() in libstout.

Review: https://reviews.apache.org/r/11108
Shuffle offers before sending to frameworks.

This helps ensure fair allocation of resources.

Review: https://reviews.apache.org/r/11121
Added --enable-debug configure flag.

Review: https://reviews.apache.org/r/11605
Slave ping timeout may be increased by flags.

Review: https://reviews.apache.org/r/12885
Turned down noise from ZooKeeper.

Review: https://reviews.apache.org/r/11127
Fixed broken build on GCC 4.8 due to -Wall.

With GCC 4.8, Boost fails to build from '-Wunused-local-typedefs', which
is implied by '-Wall'.

Review: https://reviews.apache.org/r/11974/
Use hostname instead of IP for redirection.

In the Web UI, the redirection does not work for hosts where the local
interface address is not publicly accessible.  For example, with EC2 the
redirection will not work.

This change writes the Master's hostname into ZooKeeper, thereby
allowing the redirection to work with private IP addresses.

Review: https://reviews.apache.org/r/11975/
On recovery fail, do rm -rf /tmp/mesos/meta.
Use libstout cache for slave completedFrameworks.

The list of retired (completed) frameworks is now a stout cache rather
than a boost circular buffer.  There are now key iterator accessors for
the cache class, and a removal member.

Review: https://reviews.apache.org/r/13599
Added a slave load hint for scheduler sorting.

With this change, the offers will include a slave load hint based on the
5 minute load average for that slave (returned from getloadavg()).
Frameworks can use this hint to select among otherwise similar offers,
as an optimization.

Review: https://reviews.apache.org/r/13675