Modify data-xfer semantics: it is no longer necessary to call one of
request(), seek() or deliver_xxx() in order to start the data flow.
Autonomous generators must be genuinely autonomous (having their own
process), or otherwise arrange to be called. TCP does this by
starting the retry timer immediately.
Add some debugging statements.
Do not hold self-references. This then avoids the problem of having to
ensure that we only drop our self-reference exactly once.
To maintain the guarantee that an object won't go out of scope
unexpectedly while one of its event handlers is being called, the
event-calling functions now automatically obtain and drop extra
references.