Adding an ability to specify batches num for online migrations.

- By default migrations would use 50 chunks per round.
  We can specify larger number to speed it up.

Prod-Related: PROD-33940
Change-Id: I3d2d3f4590b3a303be55ae76f8e3e701b24e4563
(cherry picked from commit b139f14434843b675ca710cede5bf76e31011414)
2 files changed