Reference Registry

Creates a hash ( named disref_{{process_id}} ), which tracks all the references associated with a particular process.  
Since redis executes this in a single thread of execution this should be fine. Otherwise we would need non-reentrant locks to ensure no two threads were using a single `Process` at a time (which should be an obvious no-no, but we're only human). (https://docs.python.org/2/library/threading.html#threading.Lock)