[Needs review.] Add a BEGIN_DIR relay cell type for an easier
 in-protocol way to connect to directory servers through Tor.
 Previously, clients could only connect to director servers over Tor
 from exit nodes, but couldn't get directory information anonymously
 from a non-exit cache without getting a directory server involved.
 
 This needs testing, and needs client-side code to actually exercise it.
 


svn:r8527