send the end cell when we realize we're going to end,

not when we're closing the stream.

this lets us put a payload in the end cell if we want to,
to describe why we're closing the stream.

there are still some places where we don't send the end cell
immediately. i need to track them down. but it's a low priority,
since i've made it send the end cell when we close the stream if
we haven't already sent it.


svn:r640