Workers should be forked right at the start, to avoid forked processes
containing allot of irrelevant objects.
Instead of depending on forked memory to receive the graph, workers
will load the graph from the database once it is available.
APIs should be refused until vitrage-graph is fully loaded. This was
previously supported because API processes were only started when ready.
GraphInit waits untill all worker processes are started and listen on
the multiprocessing queue.