cluster reindexing service could set a default scroll setting

Background

scroll setting was recently introduced to cluster reindexing service in the MR mentioned below

The following discussion from !156519 (merged) should be addressed:

• @johnmason started a discussion: (+2 comments)

  thought (n/b): ah, I see scroll is nil here! Just want to be sure that doesn't blow up in the test cases.

Proposal

use a default for the scroll setting, we could try for 1h or 2h or make it required by callers