Segmented fetch in courier

This pr moves the segmented fetch from discover to the courier. It also makes starting an iteration of the loopers search handler before the previous itteration completes [a fatal error](https://github.com/spenceralger/kibana/commit/8ae871c0c907336c512166a5cc40a166f03038dc#diff-679f22f75c2bbe5ca3979d9dd0e677dfR18)