More bonus for bestMoves on past PV nodes

It looks like it is important to keep past PV (ttPv) nodes as close as possible to current PV nodes.
Credits to Mark Tenzer (31m059) & Stefan Geschwentner who first tried ideas on ttPv nodes.

STC:
https://tests.stockfishchess.org/tests/view/5e2ff5efab2d69d58394fd52
LLR: 2.95 (-2.94,2.94) {-1.00,3.00}
Total: 13302 W: 2647 L: 2507 D: 8148
Ptnml(0-2): 237, 1540, 2956, 1632, 260

LTC:
https://tests.stockfishchess.org/tests/view/5e2fff38ab2d69d58394fd55
LLR: 2.95 (-2.94,2.94) {0.00,2.00}
Total: 15797 W: 2137 L: 1960 D: 11700
Ptnml(0-2): 96, 1443, 4628, 1547, 130

closes https://github.com/official-stockfish/Stockfish/pull/2529

bench: 5545845