'Request evaluation of the given completion.
"evalr" is the Evaluator instance.
The manager has an evaluation thread on which this evalr will be
scheduled. Only one request is ever eval\'d at one time. A new
request will cause an existing on to be aborted and requests made in
the interim will be trumped by this new one.
Dev Notes:
- XXX Add a timeout to the put and raise error on timeout?'

def request_eval(self, evalr):

    self.put((evalr, False))
