Zoekt: Add replicas columns

Background

This is the issue to allow specifying the number of replicas for Zoekt.

Proposal

• Add a new application setting zoekt_number_of_replicas or zoekt_replicas to specify the number of replicas per namespace (default: 1) Add a note in the description that this setting not active yet (until we finish other issues in this epic)
• Add a nullable replicas column to the zoekt_enabled_namespaces table to allow overriding the default number of replicas per each namespace