Previously, the firestorm.Batch() would notify the lookuper goroutine
on a document by document basis. If the lookuper input channel became
full, then that would block the firestorm.Batch() operation.
With this change, lookuper is notified once, with a "batch" that is an
[]*InFlightItem.
This change also reuses that same []*InFlightItem to invoke the
compensator.MutateBatch().
This also has the advantage of only converting the docID's from string
to []byte just once, outside of the lock that's used by the
compensator.
Micro-benchmark of this change with null-firestorm bleve-blast does
not show large impact, neither degradation or improvement.
