Updated the slave to send terminal tasks for terminated executors
during re-registration.

The slave must send terminal unacknowledged tasks for terminal
executors so that the master can correctly reconcile tasks when
the slave re-registers.

Otherwise, the master may see a missing task
(terminal, unacknowledged) on the slave and incorrectly inform
the framework via TASK_LOST, per MESOS-1388.

Review: https://reviews.apache.org/r/21991