Added libprocess metrics to master/slave '/stats.json'.

Review: https://reviews.apache.org/r/20373
Added Containerizer::containers() to get set of containers.

Review: https://reviews.apache.org/r/20428
Added metrics to the registrar.

Review: https://reviews.apache.org/r/20581
Fixed containerizer::Termination use in SlaveRecoveryTest.

Review: https://reviews.apache.org/r/20639
Added timeouts to registrar.

Review: https://reviews.apache.org/r/20374
Fixed a warning in ReservationAllocatorTest.ResourcesReturned.

Review: https://reviews.apache.org/r/20633
Fixed a CHECK failure in Statistics.
Introduced exponential backoff for slave registration retries.

Review: https://reviews.apache.org/r/20502
Fixed a compiler warning in Statistics.