BitBucket Cloud: Fix migration timeouts

Problem

As described in #411534 (closed), BitBucket Server and the BitBucket Cloud importers one worker to migrate the entire project. When migrating large projects, we often experience migration timeouts.

Proposal

We should improve reliability of our BitBucket Cloud importer by breaking the importer into multiple workers.

On BitBucket Server, we refactored it to use the same architecture used in GitHub Importer.