vp9: set kf_max_dist to a reasonable default (128)

the same as vp8, with the same reasoning from:
2a0d7b1a Reduce the default kf_max_dist to 128.

see also:
https://trac.ffmpeg.org/ticket/4904
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=815673

+ restore vpxenc behavior of taking the library default rather than
  forcing 5s

This change also exposes an issue with one-pass svc in cbr mode, keep
the old default in datarate_test.cc for now.

Change-Id: Id6d1244f42490b06fefc1a7b4e12a423a1f83e88