depth: Limit 6th and 7th move depth to 15

Because if considerMobility is enabled, 6th and 7th
is too slow, so need to limit it. But cannot limit
less than 15, otherwise, c5 d5 b4 f4 d6 d2,
next move, white will place a lose point.

Now the performance only reduce 12%.

src/mills.cpp

@@ -456,8 +456,8 @@ Depth get_search_depth(const Position *pos)
     if (!gameOptions.getDeveloperMode()) {
         if (pos->phase == Phase::placing) {
             const Depth placingDepthTable[] = {
-                  +1, 1, +1,  1,     /* 0 ~ 3 */
-                  +3, 0, +0,  0,     /* 4 ~ 7 */
+                  +1,  1,  +1,  1,   /* 0 ~ 3 */
+                  +3, 15, +15,  0,   /* 4 ~ 7 */
                   +0, 0, +0,  0,     /* 8 ~ 11 */
                   +0, 0, +0,  0,     /* 12 ~ 15 */
                   +0, 0, +0,  0,     /* 16 ~ 19 */