Send END cells on bufferevent tunneled directory conns

Our old code correctly called bufferevent_flush() on linked
connections to make sure that the other side got an EOF event... but
it didn't call bufferevent_flush() when the connection wasn't
hold_open_until_flushed.  Directory connections don't use
hold_open_until_flushed, so the linked exit connection never got an
EOF, so they never sent a RELAY_END cell to the client, and the
client never concluded that data had arrived.

The solution is to make the bufferevent_flush() code apply to _all_
closing linked conns whose partner is not already marked for close.