While it really would be desirable to allow stream destruction during on_read()
callbacks, this does not work anymore since e49b2998. Until we have a proper
solution for this issue, use asynchronous disconnects for the only user doing
so.
Fixes#518.
While this will complicate the implementation of streams not based on a fd,
it allows us to unleash the full power of FILE based convenience functions.
Martin Willi