11684: ArvadosFile.flush() now check if it is the only owner of a bufferblock before...

11684: ArvadosFile.flush() now check if it is the only owner of a bufferblock before deleting it, so the extra argument is not required.
On commit_all(), always check if the owner attribute is an instance of ArvadosFile before calling flush()
Fixed a couple tests that were mocking bufferblock.owner so that they work with this new behavior.