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
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
Increase web UI reload delay.
Fixed some task reconciliation cases.

Case 1:

If a slave is known but the task cannot be found, we should assume that
the task has been lost.  It's possible that the following events
occurred:

 1) Framework disconnected from master
 2) Master terminated framework's tasks
 3) Framework reconnects to master, and (incorrectly) assumes tasks are
 still running

Case 2:

If a framework loses track of running tasks, the master should inform
the framework of which tasks it knows to be running, in addition to any
which have had a state change.

Review: https://reviews.apache.org/r/15745