Don't accept offers on the same host/port.

We'll try to find a port combination that hasn't already been used to
launch a TaskTracker.

Review: https://reviews.apache.org/r/11161/
Call `post-reviews' with --guess-fields argument.

But only do so with the initial creation of the review.

Review: https://reviews.apache.org/r/11159
Ignore errors from post-review.

When updating existing reviews, post-review returns errors erroneously.

Review: https://reviews.apache.org/r/11160
Write Mesos JobTracker stats to state file.

Review: https://reviews.apache.org/r/11165/