Added checkpointing ability for example frameworks.

Review: https://reviews.apache.org/r/13398
Fixed slave to not re-register with master in cleanup mode.

Review: https://reviews.apache.org/r/13442
Improved the allocator to not offer non-checkpointing slave resources
to checkpointing frameworks.

Review: https://reviews.apache.org/r/13407
Fixed slave to always delete the latest slave symlink on
shutdown or cleanup.

Review: https://reviews.apache.org/r/13392
Added fs::size() to calculate the total disk space
and killed fs::available().

Review: https://reviews.apache.org/r/13394
Fixed slave to offer total disk by default.

Review: https://reviews.apache.org/r/13395
Changed slave state recovery to ignore absence of files as safe.

Review: https://reviews.apache.org/r/13443
Fixed master to properly handle TASK_LOST updates generated by it.

Review: https://reviews.apache.org/r/13446
Fixed slave to not recover terminated executors.

Review: https://reviews.apache.org/r/13450
Fixed cgroups isolator to do setsid on the executor process.

Review: https://reviews.apache.org/r/13449
Improved task validation error messages in master.

Review: https://reviews.apache.org/r/13454
Implemented a LinkedHashMap that preserves the insertion
order of keys of a hashmap.

Review: https://reviews.apache.org/r/13464
Fixed slave to use LinkedHashMap to store tasks.

Review: https://reviews.apache.org/r/13485
Fixed executor driver to store tasks and updates in LinkedHashMap.

Review: https://reviews.apache.org/r/13486
Fixed slave to not re-register with Command Executors.

Review: https://reviews.apache.org/r/13487
Fixed slave to always call Isolator::resourcesChanged() when an
executor re-registers.

Review: https://reviews.apache.org/r/13503
Fixed master to properly reconcile completed frameworks when
slave re-registers.

Review: https://reviews.apache.org/r/13508
Fixed status update manager to properly handle duplicate updates.

Review: https://reviews.apache.org/r/13520