Add a case that blow up google_dense_hash_map

[JTJL]

Here is a case that google_dense_hash_map use much more time than stl_unordered_map.
Key is generated simply by (i << 10) | (i & 1023) where i increase from 1.

gapinsert,1000000,stl_unordered_map,53964800,0.142443
gapinsert,1000000,google_dense_hash_map,47181824,5.537036
gapinsert,1100000,stl_unordered_map,65916928,0.128728
gapinsert,1100000,google_dense_hash_map,80736256,7.489668
gapinsert,1200000,stl_unordered_map,69160960,0.135847
gapinsert,1200000,google_dense_hash_map,80736256,8.094367
gapinsert,1300000,stl_unordered_map,72269824,0.141389
gapinsert,1300000,google_dense_hash_map,80736256,8.793521
gapinsert,1400000,stl_unordered_map,75513856,0.150329
gapinsert,1400000,google_dense_hash_map,80736256,9.628102
gapinsert,1500000,stl_unordered_map,78757888,0.229699
gapinsert,1500000,google_dense_hash_map,80736256,10.818993
gapinsert,1600000,stl_unordered_map,81866752,0.168996
gapinsert,1600000,google_dense_hash_map,80736256,11.671272
gapinsert,1700000,stl_unordered_map,85110784,0.243634
gapinsert,1700000,google_dense_hash_map,80736256,12.563268
gapinsert,1800000,stl_unordered_map,88354816,0.264759
gapinsert,1800000,google_dense_hash_map,80736256,13.661313
gapinsert,1900000,stl_unordered_map,91463680,0.271045
gapinsert,1900000,google_dense_hash_map,80736256,14.876894
gapinsert,2000000,stl_unordered_map,94707712,0.194570
gapinsert,2000000,google_dense_hash_map,80736256,16.059781