Temporarily disabled HealthcheckTest.ConsecutiveFailures test due to
MESOS-1613.
Fixed allocator to do allocations per slave rather than per framework.

Review: https://reviews.apache.org/r/24356
Retry freeze in cgroups TasksKiller to workaround MESOS-1689.

Review: https://reviews.apache.org/r/24511
Added overloads to 'process::defer()' to dispatch lambda function
to a specific pid.
Added authorization support for "/shutdown" endpoint and updated
ACLs format.
Allowed cgroups mem isolator to limit swap by setting
memory.memsw.limit_in_bytes.

Review: https://reviews.apache.org/r/24316
Added a JSON::Object::find.

Review: https://reviews.apache.org/r/24541
Changed Try::error and Result::error to return const references.

Review: https://reviews.apache.org/r/18489
Added missing 'defer' helpers.

Fixes compilation issue when building with gcc 4.4, see MESOS-1692 for
more details.

Review: https://reviews.apache.org/r/24569