Make migration pause time a configurable value

What does this MR do?

Related to #326840 (closed)

Batched background migration jobs sleep for a small interval between execution of each batch they process. Previously, that pause time had been a fixed value of 0.1 seconds. However, when processing a large number of small batches, that sleep time can dominate the total runtime of the job.

This MR adds a new column to batched_background_migrations and batched_background_migration_jobs, and allows this attribute to be reconfigured on the fly.

Database Migrations

Up

$ bundle exec rails db:migrate:up VERSION=20210413155324
== 20210413155324 AddPauseSecondsToBatchedBackgroundMigrations: migrating =====
-- add_column(:batched_background_migrations, :pause_ms, :integer, {:null=>false, :default=>100})
   -> 0.0043s
== 20210413155324 AddPauseSecondsToBatchedBackgroundMigrations: migrated (0.0043s) 

$ bundle exec rails db:migrate:up VERSION=20210414045322
== 20210414045322 AddPauseSecondsToBatchedBackgroundMigrationJobs: migrating ==
-- add_column(:batched_background_migration_jobs, :pause_ms, :integer, {:null=>false, :default=>100})
   -> 0.0029s
== 20210414045322 AddPauseSecondsToBatchedBackgroundMigrationJobs: migrated (0.0029s) 

Down

$ bundle exec rails db:migrate:down VERSION=20210414045322
== 20210414045322 AddPauseSecondsToBatchedBackgroundMigrationJobs: reverting ==
-- remove_column(:batched_background_migration_jobs, :pause_ms, :integer, {:null=>false, :default=>100})
   -> 0.0024s
== 20210414045322 AddPauseSecondsToBatchedBackgroundMigrationJobs: reverted (0.0058s) 

$ bundle exec rails db:migrate:down VERSION=20210413155324
== 20210413155324 AddPauseSecondsToBatchedBackgroundMigrations: reverting =====
-- remove_column(:batched_background_migrations, :pause_ms, :integer, {:null=>false, :default=>100})
   -> 0.0026s
== 20210413155324 AddPauseSecondsToBatchedBackgroundMigrations: reverted (0.0039s) 

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