Use an up-to-date stem
Get the exit policy from descriptors when available

 * Some complex exit policies are summarized as reject 1-65535. Since
   the consensus only contains the summary, calls to can_exit_to
   incorrectly report false.

 * Some examples from a current consensus:

     03FF94D9E5001DD2290BC3B19FA7F59CE1E30279
     10C0C888E4812BEFCD7B9449E5310A022FD1325B
     296B2178FD742AB35AB20C9ADF04D5DFD3D407EB
     65C35C03571307D7546D6978605A6B11B473F6EE
     78C7C299DB4C4BD119A22B87B57D5AF5F3741A79
     8887213ED36CE2810E4162E6254D3CF7324C3A1B
     C4D41BFB33993F9D64699726A445283CE307E30C
     CB2BEB364E07CF431819F6C5349555425C60C6F3
     D897E7055BF702763BB88442F73D46CF7DBC0B6F

 * Here we parse the cached-descriptors, when available, and use the
   full exit policy if present for a relay.
Merge pull request #4 from arlolra/is_exiting_allowed

Get the exit policy from descriptors when available