restore -DNDEBUG
pondering: step 1
SearchLimits::ponder
rename shelter to shield
perft interactive command
Merge branch 'master' into ponder
retire PV from TT stuff
backup the PV through the search
Finally DiscoCheck can display a correct PV. This will make pondering possible.
And code is significantly simpler and shorter than the TT from PV stuff.

Method used is the one exposed by Bruce Moreland on his tutorial. Thanks to Don Dailey for explanation how to extend the simple case presented by Bruce Moreland to a search with many possible exit points (simply pv[0]=0 at PV nodes at the entry point of the search, which avoids backing up garbage, avoids having to terminate the PV at every possible exit point).

=> next step: extend this to the qsearch