Unlock repository during collection import

Moved from: https://github.com/pulp/pulp_ansible/issues/1105 since this needs to be fixed in galaxy.

Right now collection imports lock the entire repository that the collection is being imported into (https://github.com/ansible/galaxy_ng/blob/master/galaxy_ng/app/api/v3/viewsets/collection.py#L65). This makes it so that only one collection can be imported at a time. This is problematic because the import task can take a long time, and doesn't need to maintain a lock on the repository for the full duration of the import.

Describe the solution you'd like
Remove the repository lock during collection import and have the collection import launch a subtask (with the repository lock) to move the collection into the selected repository after the import is successful.

Additional context
This is needed for repository management in galaxy_ng to get rid of inbound repositories.