support multiple logs and administrator-provided scts

filesystem layout:

SCT storage on disk:

<rootdir>/<fingerprint>/hostname_port_uri.sct
  SCT for cert with this fingerprint
  from this log (could be any number
  of these)

<rootdir>/<fingerprint>/<anything>.sct
  SCT maintained by the administrator
  (file is optional; could be any number
  of these)

<rootdir>/<fingerprint>/collated
  one or more SCTs ready to send
  (this is all that the web server
  processes care about)