Time management: move faster if PV is stable

Move faster but compensate by allocating more
time when the best move changes.

Passed short TC at 15+0.05
LLR: 2.93 (-2.94,2.94)
Total: 13895 W: 3030 L: 2882 D: 798

Long TC at 60+0.05
LLR: 2.96 (-2.94,2.94)
Total: 9266 W: 1777 L: 1624 D: 5865

At time increment 30+0.5
LLR: 2.96 (-2.94,2.94)
Total: 6703 W: 1238 L: 1134 D: 4331

And at fixed game number, longer TC 120+0.05
ELO: 5.17 +-2.8 (95%) LOS: 100.0%
Total: 19306 W: 3378 L: 3091 D: 12837

bench: 4728533

src/timeman.cpp

@@ -76,10 +76,9 @@ namespace {
 }
 
 
-void TimeManager::pv_instability(int curChanges, int prevChanges) {
+void TimeManager::pv_instability(float bestMoveChanges) {
 
-  unstablePVExtraTime =  curChanges  * (optimumSearchTime / 2)
-                       + prevChanges * (optimumSearchTime / 3);
+  unstablePVExtraTime = int(bestMoveChanges * optimumSearchTime);
 }