v0.30.0-rc2

In this release:

• WDL conversion for more realistic WDL score and contempt. Adds an Elo based
  WDL transformation of the NN value head output. Helps with more accurate play
  at high level (WDL sharpening), more aggressive play against weaker opponents
  and draw avoiding openings (contempt), piece odds play. There will be a blog
  post soon explaining in detail how it works.
• A new score type WDL_mu which follows the new eval convention, where +1.00
  means 50% white win chance.
• Simplified to a single --draw-score parameter, adjusting the draw score from
  white's perspective: 0 gives standard scoring, -1 gives Armageddon scoring.
• Updated describenet for new net architectures.
• Added a first-move-bonus option to the legacy time manager, to accompany
  book-ply-bonus for shallow openings.
• Changed mlh threshold effect to create a smooth transition.
• Revised 'simple' time manager.
• A new spinlock implementation (selected with --search-spin-backoff) to help
  with many cpu threads (e.g. 128 threads), obviously for cpu backends only.
• Some assorted fixes and code cleanups.