Store slice multiplier and max slices running for reindexing in database

What does this MR do?

Related to #294210 (closed)

backend work required to allow admins to customize the reindexing process by choosing the max # of running slices and how many slices for the reindexing process. The frontend work will be completed in: !60730 (merged)

Highlight of changes:

• Store two new values in elastic_reindexing_tasks: Slice multiplier and Max slices running
• Set default values for both fields if they are not provided (or set to nil)
• Update specs

Database

up

bundle exec rails db:migrate:up VERSION=20210430154631
== 20210430154631 AddSliceMultiplierAndMaxSlicesToElasticReindexingTask: migrating
-- add_column(:elastic_reindexing_tasks, :max_slices_running, :integer, {:limit=>2, :default=>60, :null=>false})
   -> 0.0068s
-- add_column(:elastic_reindexing_tasks, :slice_multiplier, :integer, {:limit=>2, :default=>2, :null=>false})
   -> 0.0013s
== 20210430154631 AddSliceMultiplierAndMaxSlicesToElasticReindexingTask: migrated (0.0082s)

down

 bundle exec rails db:migrate:down VERSION=20210430154631
== 20210430154631 AddSliceMultiplierAndMaxSlicesToElasticReindexingTask: reverting
-- remove_column(:elastic_reindexing_tasks, :slice_multiplier, :integer, {:limit=>2, :default=>2, :null=>false})
   -> 0.0110s
-- remove_column(:elastic_reindexing_tasks, :max_slices_running, :integer, {:limit=>2, :default=>60, :null=>false})
   -> 0.0009s
== 20210430154631 AddSliceMultiplierAndMaxSlicesToElasticReindexingTask: reverted (0.0148s)

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