EQL: remove zero-downtime migration workflow

[tobyhede]

Make zero downtime migration workflow optional and no longer required eql_v2.migrate_config() and eql_v2.activate_config() as the default.

Config is now immediately active unless add_search_config is passed a migrating param of true

SELECT eql_v2.add_search_config('users', 'name', 'match', migrating => true);