Block flushing of Kf.PersonaStore on any pending file operations

If a file operation is still underway, don't allow the Kf.PersonaStore to be
finalized until it's finished. This prevents libfolks from being closed before
changes to relationships.ini have been written out. We also prevent multiple
file operations from happening (pseudo-) concurrently, by cancelling any
pending operation when we schedule a new one.