Slice filter: never run subrequests when main request is buffered.

Slice filter: never run subrequests when main request is buffered.

With main request buffered, it's possible, that a slice subrequest will send
output before it.  For example, while main request is waiting for aio read to
complete, a slice subrequest can start an aio operation as well.  The order
in which aio callbacks are called is undetermined.