ceph: async write new measures
ceph: async write new measures

we should use async writes since we're not using threads anymore.

testing on small ceph cluster and posting 20metrics/POST results in
~2x better write performance.

Change-Id: Ic451e6ea874a2a3695d3d148b1c05ecc54aa473b

sileht/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi