This one is a little tricky.  Our BEGIN_DIR implementation has a
 problem: the dirserv conns will decide they can flush all their data
 immediately, since the edge_conns will read greedily.
 
 For our 0.1.2 workaround, we track which or_conn a bridged dirserv
 conn is attached to, and stop writing when its outbuf is too full, and
 start writing again when the or_conn's outbuf empties out a little.
 
 This requires a bit of pointer management.  Let's hope it works.
 


svn:r9432