On file Remove
- cancel any writebacks in progress
- ignore error message deleting non existent file if file was in the
process of being uploaded
Writeback
- Don't transfer the file if it has disappeared in the meantime
- Take our own copy of the file name to avoid deadlocks
- Fix delayed retry logic
- Wait for upload to finish when cancelling upload
Fix race condition in item saving
Fix race condition in vfscache test
Make sure we delete the file on the error path - this makes cascading
failures much less likely
