Fix a bufferevent-related bug that killed tunneled dirserv conns

Fix a bufferevent-related bug that killed tunneled dirserv conns

Because tunneled connections are implemented with buffervent_pair,
writing to them can cause an immediate flush.  This means that
added to them and then checking to see whether their outbuf is
empty is _not_ an adequate way to see whether you added anything.
This caused a problem in directory server connections, since they
would try spooling a little more data out, and then close the
connection if there was no queued data to send.

This fix should improve matters; it only closes the connection if
there is no more data to spool, and all of the spooling callbacks
are supposed to put the dirconn into dir_spool_none on completion.

This is bug 3814; Sebastian found it; bugfix on 0.2.3.1-alpha.