Implemented array subscript lookup in JSON::Object::find.

Review: https://reviews.apache.org/r/26634
Fixes necessary due to JSON::Array::values change to std::vector.

Review: https://reviews.apache.org/r/26635
Mark running tasks killed during framework shutdown.

When a framework is shut down e.g. by calling driver.stop() from the
scheduler, running tasks are marked KILLED before migrating them to
completed.

Review: https://reviews.apache.org/r/25250
Fixed a spelling mistake.
Introduced a FutureResult action.

The FutureResult action enables capturing the result from an inner
action, e.g., an Invoke* action.

Review: https://reviews.apache.org/r/27199
PortMappingIsolator: Swap TX/RX in statistic collection in usage

statistics collected on the host end of veth pair have reversed RX/TX of those in the container.

Review: https://reviews.apache.org/r/27154
Added a check in routing library due to a bug in libnl.

Review: https://reviews.apache.org/r/27105
Added documentation for egress rate limit control.

Review: https://reviews.apache.org/r/26423