Delay search indexing to work around transaction

Scheduled publishing is currently performed every 15 minutes across all backend machines (keeps all machines equal and introduces redundancy into the process) using a rake task. Each due edition is published by `EditionPublishingWorker`, which performs the action with a transaction lock so that only one backend machine will actually succeed if more than one attempts to publish the same edition. The issue is that whilst the transation is in progress, the edition will remain in a "scheduled" state in the database. And because search indexing is performed asynchronously by sidekiq, it is sometimes the case that the search indexing job will be picked up and attempted before the transaction can complete. This results in the edition not being indexed, as `SearchIndexAddWorker` will not index a non-"published" edition.

This change introduces a 10 second delay to the search indexing job to give the transaction enough time to complete.

Note that this is a temporary measure to get scheduled editions indexing reliably. A story is to be spun up for a fuller solution that doesn't rely on a transaction lock when publishing scheduled editions.