Add flags to Board::Cell.
Mark dead cells and IsCellDead() skips those previously marked.
Fix bugs in Board::IsCellDead().
Refactor code in YUctSearch.
Track cells that need to have their weights recomputed.
Add 'dirty_cells' gtp command.
Mark more cells as dirty during a merge.
Compute carrier while computing group.

Fixed bug in MergeBlocks: edges weren't transfering to 'to'.
Fixed bug in GroupSearch: blocks marked as potential in other
searches were never considered again (!).