Migrate GitGarbageCollectWorker to Gitaly

What does this MR do?

Migrates GitGarbageCollectWorker to Gitaly

3 new feature-flags:

• :gitaly_garbage_collect
• :gitaly_repack_full
• :gitaly_repack_incremental

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitaly#398 (closed)