This allows to pass any GraphChain to the Graph to process.
It removes the need to use a mutex to swap two dedicated
chains (setup-chain <> active-chain, pending-chain).
Also various special cases pertaining to graph interaction
while auditioning and route-deletion can be removed.
This also unconditionally creates a graph-thread for GraphChains
to be processed, even if the main callback uses a special-cased
sorted RouteList if there is only one process thread.
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