This adds a new implementation of the function engine that runs the DAG
function graph. This version is notable in that it can run a graph that
changes shape over time. To make changes to the same of the graph, you
must use the new transaction (Txn) system. This system implements a
simple garbage collector (GC) for scheduled removal of nodes that the
transaction system "reverses" out of the graph.
Special thanks to Samuel Gélineau <gelisam@gmail.com> for his help
hacking on and debugging so much of this concurrency work with me.
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