Add value pessimism parameter

[Ttl]

This parameter controls the number of visits that initial Q should have in later Q updates. I clopped the best value at very quick time controls to be around 0.6.

At longer time controls and other values being default it seems to be improvement:

Score of lc0_fpu_visits_35258 vs lc0_master_35258: 45 - 25 - 330  [0.525] 400
Elo difference: 17.39 +/- 14.16, LOS: 99.16 %, DrawRatio: 82.5 %

Network 35258, TC: 10s+0.5s, GPU GTX 1080 Ti, 5 man TBs and all other parameters defaults.

[Ttl]

Some more tests with the latest commit. Hopefully no functional changes. It uses the pessimistic Q value only for picking the positions for the batch. The reported score is not affected anymore.

Score of lc0_value_pessimism_06 vs lc0_master: 92 - 74 - 634  [0.511] 800
Elo difference: 7.82 +/- 10.95, LOS: 91.88 %, DrawRatio: 79.3 %

800 nodes, MinibatchSize=32, Slowmover=0 

Score of lc0_value_pessimism_06 vs lc0_master: 68 - 60 - 272  [0.510] 400
Elo difference: 6.95 +/- 19.25, LOS: 76.02 %, DrawRatio: 68.0 %

800 nodes, MinibatchSize=32 

Score of lc0_value_pessimism_06 vs lc0_master: 92 - 74 - 634  [0.511] 800
Elo difference: 7.82 +/- 10.95, LOS: 91.88 %, DrawRatio: 79.3 %

1+0.5s, GTX 1080 Ti

The later tests are all positive, but all are inside the error bounds. Probably still needs more tests to make sure that this gains Elo.

[Naphthalin]

This is an interesting approach (and somehow similar in general to the initial AlphaGo paper, where the NN evaluation was treated as 100 MC rollouts). @Ttl do you still think this is a sensible approach, and if yes update the PR and test with recent nets?