Fix for bad behavior when using TailableCursor.
Fix for bad behavior when using TailableCursor.

`Database.MongoDB.Query.next` and `nextBatch` prefetch a promise of the next
batch of documents from the server whenever the current batch has been
exhausted. The following call to `next` or `nextBatch` fulfills that promise
(thereby turning it into a concrete batch of documents) and then returns one or
more documents in the batch.

The old behavior was to raise an exception if an empty batch with a nonzero
cursor ID was encountered. This is normal when using tailable cursors, so a
change was required.

Now, `Nothing` is returned with the still-live cursor ID, instead of raising
the exception. Also, prefetched empty batches with nonzero cursor IDs are
refetched once per call to avoid stale data.

This new prefetching behavior does not affect the performance of `next`, except
when calling it repeatedly on a tailable cursor with no new data. In those
(generally avoidable) cases, each call to `next` results in two server calls
instead of one.

mongodb-haskell/mongodb,Yuras/mongodb,VictorDenisov/mongodb