- Document the correct persistent filenames and the presence of a
  magic number at the start of the file.  (Also document the
  conditions under which the cache is persistent.)

- In __init__(), seek to the correct position to read the first
  record's serialno.  It was reading the last byte of dlen plus the
  first 7 bytes of the serialno.

- In invalidate(), fix the seek call after the read(8) to actually
  seek to the status byte.  It was seeking 8 bytes too far, causing it
  to overwrite the 2nd byte of dlen instead of the status byte.