BitBucket Server: Fix migration timeouts

Problem

Both, the 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 BitBucket Server by breaking the importer into multiple workers.

We have a separate issue for introducing parallelised workers for the BitBucket Cloud importer: BitBucket Cloud: Fix migration timeouts (#412614 - closed) This is out of scope for this issue.