The previous behaviour was unsound, as it was prone to dropping
connections under (temporary) high load. The new behaviour requires that
users are well-behaved with respect to shutdown - the root listener
must be shut down before any of the child listeners are, otherwise
deadlocks may occur. This requirement seems reasonable.
When the root listener is closed, child listeners will not be closed
until all parked connections are served. This prevents losing
connections that have been read from.
This also allows moving the main test to package cmux_test, but that
will happen in a separate change.
