Remove fragmentation from mem_used stat
Remove fragmentation from mem_used stat

After extensive tests, we decided to not consider the
allocator's fragmentation overhead (mostly from the central
cache in tcmalloc) in the engine's mem_used stat at this time.
It turns out that including fragmentation overhead caused quite
heavy (possibly unnecessary) item evictions. We will revisit
this issue later.

Change-Id: Ib4658cf10e3954c73fffdd2d248354ab7635562b
Reviewed-on: http://review.couchbase.org/15827
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-on: http://review.couchbase.org/15841

owendCB/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,sriganes/ep-engine,hisundar/ep-engine,daverigby/ep-engine,daverigby/kv_engine,daverigby/kv_engine,jimwwalker/ep-engine,daverigby/ep-engine,abhinavdangeti/ep-engine,sriganes/ep-engine,daverigby/kv_engine,membase/ep-engine,membase/ep-engine,sriganes/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,membase/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,sriganes/ep-engine,hisundar/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/kv_engine,owendCB/ep-engine,teligent-ru/ep-engine