Fix harbinger file deletion race condition (resolves #1046)
Fix harbinger file deletion race condition (resolves #1046)

resolves #1046

Harbinger file deletion now occurs only with the cache lock held.
Added logic to handle a race case between downloading jobs
attempting to delete the harbinger of a prematurely terminated
file store operation.

BD2KGenomics/slugflow,BD2KGenomics/slugflow