Add 'value' and 'units' methods to Duration subclasses.

Review: https://reviews.apache.org/r/20922
Used Duration subclasses to allow Timer users to specify units.

Review: https://reviews.apache.org/r/20923
Added template parameter to Timer.

Review: https://reviews.apache.org/r/20925
Temporarily disabled SlaveRecoveryTest.RestartBeforeContainerizerLaunch due to MESOS-1273.
Improved log messages in master.

Review: https://reviews.apache.org/r/20932
Ignored the log write request to a position if it has already been
learned.

Review: https://reviews.apache.org/r/20934