inspiral.py: reorganize collection of zero-lag LRs

- a histogram of likelihood ratios assigned to zero-lag candidates is
  required to implement the low-significance extinction model.  this patch
  reorganizes the source and sink of that information to be a separate file
  rather than piggy backing the data on the candidate parameter PDF file
  supplied to the trigger generator, e.g., during online running.

- immediately, this allows the statistics uploaded to gracedb along with
  each candidate to contain the actual ranking statistic PDF data used to
  rank the event, whereas prior to this patch it contains a mix of the
  candidate parameter PDFs and whatever was in the zero-lag histogram data
  supplied on input.  going forward, keeping distinct, unrelated, pieces of
  information in distinct places with different names that get supplied to
  programs using distinct command line options will prevent confusion over
  what is being stored where.

- refs #2429