[Couchstore to Magma migration] : Failover way of migration to be supported

As per documentation : https://github.com/couchbase/couchbase-operator/blob/0581937ae13ff0cdd23cdcd25142cfde81c71d9f/docs/user/modules/ROOT/pages/reference-bucket-backend-migration.adoc

Couchstore to magma migration and vice versa works only with swap rebalance. Couchstore to magma migration can also be done by gracefully failing over a pod and then full recovery post that.

Ref : https://docs.couchbase.com/server/current/manage/manage-buckets/migrate-bucket.html

Operator always tries to migrate just using swap rebalance and there's no way to use the failover way to migrate.

I could not find documentation on why we are not supporting migration using failover. 

cc : Chris Malarky