Cleaned up logging in slave, status update manager and executor driver.

Review: https://reviews.apache.org/r/10708

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1476338 13f79535-47bb-0310-9956-ffa450edef68
Fixed master to consolidate tasks upon slave re-registration.

Review: https://reviews.apache.org/r/10724

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1476341 13f79535-47bb-0310-9956-ffa450edef68
Fixed the slave to do disk usage calculations on the filesystem on
which the slave work directory is mounted.

Review: https://reviews.apache.org/r/10661

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1476345 13f79535-47bb-0310-9956-ffa450edef68
Added an optional default value for Option::get().

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1477343 13f79535-47bb-0310-9956-ffa450edef68