StapledStream.send_eof should error out if you call it while send_all is pending

[njsmith]

Noticed while working on #823: normally, if you try to call send_eof on an object at the same time that another task is already calling send_all or wait_send_all_might_not_block, then send_eof should raise BusyResourceError. But StapledStream.send_eof calls self.send_stream.aclose, which has a different semantics: it interrupts any pending calls. So right now StapledStream.send_eof has the wrong semantics.

To solve this, StapledStream should detect when another task is already calling a send method, and in that case make send_eof raise an exception instead of calling self.send_stream.aclose.

I have a fix for this in my #823 branch, that I'll post shortly.