who wants to shut down a connection calls connection_mark_for_close instead
of setting marked_for_close to 1.  This automatically removes the connection
from the DNS cache if needed, sends a RELAY END cell if appropriate, and can
be changed to do whatever else is needed.

Still to do:
  - The same for circuits, maybe.
  - Add some kind of hold_connection_open_until_flushed flag, maybe.
  - Change stuff that closes connections with return -1 to use mark_for_close,
    maybe.


svn:r1145