Masters can lose track of tasks' executor IDs.

The response to the master's state endpoint sometimes doesn't include the executor id corresponding to some tasks.

Agents send to the leading master a list of tasks when reregistering. Tasks that are not started by the command executor should contain an executor id, but the following snippet can sometimes clear the executor id of tasks started by other executors: https://github.com/apache/mesos/blob/1.4.0/src/slave/slave.cpp#L1515-L1522