add latency histogram to memcached

A few requests (from moxi's point of view) are taking up to a second. Adding a histogram on memcached's side can help diagnose.

STAT 11511:default:reserved 0+100 =5 0.00%
STAT 11511:default:reserved 100+100 =147768 29.62% *******************
STAT 11511:default:reserved 200+100 =181727 66.04% ************************
STAT 11511:default:reserved 300+100 =90418 84.16% ***********
STAT 11511:default:reserved 400+100 =31442 90.46% ****
STAT 11511:default:reserved 500+100 =10765 92.62% *
STAT 11511:default:reserved 600+100 =4962 93.61%
STAT 11511:default:reserved 700+100 =3070 94.23%
STAT 11511:default:reserved 800+100 =2089 94.65%
STAT 11511:default:reserved 900+100 =1535 94.95%
STAT 11511:default:reserved 1000+100 =1180 95.19%
STAT 11511:default:reserved 1100+100 =916 95.37%
STAT 11511:default:reserved 1200+100 =856 95.54%
STAT 11511:default:reserved 1300+100 =774 95.70%
STAT 11511:default:reserved 1400+100 =757 95.85%
STAT 11511:default:reserved 1500+100 =617 95.98%
STAT 11511:default:reserved 1600+100 =540 96.08%
STAT 11511:default:reserved 1700+100 =500 96.18%
STAT 11511:default:reserved 1800+100 =441 96.27%
STAT 11511:default:reserved 1900+100 =382 96.35%
STAT 11511:default:reserved 2000+100 =414 96.43%
STAT 11511:default:reserved 2100+200 =787 96.59%
STAT 11511:default:reserved 2300+400 =1347 96.86%
STAT 11511:default:reserved 2700+800 =2087 97.28%
STAT 11511:default:reserved 3500+1600 =4049 98.09%
STAT 11511:default:reserved 5100+3200 =7525 99.60%
STAT 11511:default:reserved 8300+6400 =1406 99.88%
STAT 11511:default:reserved 14700+12800 =381 99.96%
STAT 11511:default:reserved 27500+25600 =97 99.97%
STAT 11511:default:reserved 53100+51200 =62 99.99%
STAT 11511:default:reserved 104300+102400 =9 99.99%
STAT 11511:default:reserved 206700+204800 =24 99.99%
STAT 11511:default:reserved 411500+409600 =7 100.00%
STAT 11511:default:reserved 821100+819200 =16 100.00%
STAT 11511:default:reserved 1640300+1638400=8 100.00%
STAT 11511:default:reserved 3278700+3276800=0 100.00%