BulkImports: Track Pipeline status in `BulkImports::Tracker#status`

After adding the new fields to BulkImports:Tracker (#323382 (closed)), now we can add the code to track the current status of an individual BulkImports::Pipeline. For that, it's proposed to

• Pass the BulkImports::Tracker to the pipeline, via BulkImports::Pipeline::Context;
• Update the BulkImports::Tracker#status when finishing/failing/skipping a Pipeline in the BulkImports::Pipeline::Runner;