Right now, we only consider sending stream-level SENDME cells when we
have completely flushed a connection_edge's outbuf, or when it sends
us a DATA cell.  Neither of these is ideal for throughput.

This patch changes the behavior so we now call
connection_edge_consider_sending_sendme when we flush _some_ data from
an edge outbuf.

Fix for bug 2756; bugfix on svn r152.