histogram: check for duplicates when merging
histogram: check for duplicates when merging

Improves performance because it's not necessary to caculate the length
of the histogram on each insert.
scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris