objecter: add op latency stat and in-flight count

objecter: add op latency stat and in-flight count

this is done in order to better estimate the origin of the end-to-end
operation latency. and see how much of it is due to the objecter and how
much of it is due to the client that uses it (e.g. the RGW)

Signed-off-by: yuval Lifshitz <ylifshit@redhat.com>