Have `find_route` take a `NetworkGraph` instead of a `ReadOnly` one

Have `find_route` take a `NetworkGraph` instead of a `ReadOnly` one

Because downstream languages are often garbage-collected, having
the user directly allocate a `ReadOnlyNetworkGraph` and pass a
reference to it to `find_route` often results in holding a read
lock long in excess of the `find_route` call. Worse, some languages
(like JavaScript) tend to only garbage collect when other code is
not running, possibly leading to deadlocks.