Add TaskState class to worker.py #4097

Currently we record state in the Worker class with a variety of dictionaries that hold information like the function to be run, the type of the output, the status, the dependencies that we're waiting on, the number of bytes of the output, and so on. We might consider turning all of these collections into a single collection of a TaskState type.

We did this transformation in the Scheduler a while ago see scheduler.py::TaskState and that seems to have been a good choice. We should probably repeat it for the worker. Maybe something like the following (incomplete)

class TaskState:
    def __init__(self, key, run_spec):
        self.key = key
        self.dependencies = ()
        self.dependents = ()
        self.duration = None
        self.priority = None
        self.state = None
        self.dep_state = None
        self.who_has = None
        self.nbytes = None
        self.resource_restrictions = None
        self.actor = None
        self.exception = None
        self.traceback = None
        self.type = None

This has the advantage of being a good learning task