DOC: document the listener state transitions
This was done by reading all the code affecting a listener's state,
hopefully it will save some time in the future.
diff --git a/doc/internals/listener-states.fig b/doc/internals/listener-states.fig
new file mode 100644
index 0000000..23ec87d
--- /dev/null
+++ b/doc/internals/listener-states.fig
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