Refactor calling of offer evaluation

[ssalinas]

While looking into some odd offer issues, I noticed that our invocations of checking offers differed slightly between the SchedulerImpl and the SchedulerPoller. This does two things:

• Consolidates all offer evaluation logic for new and cached offers in the SingularityMesosOfferScheduler class, so the same logic is called in all places
• Moves the drainPendingQueue operation to only the scheduler poller, since there is no need to call it on every offer run (which can be multiple times a second), making offer evaluation a bit more efficient
• Add logging on all possible offers that were used to launch a task so we have a better record of all of them

[baconmania]

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