Reviewer Andy: Add latency to access logs

Andy

Can you review my change to add request latency to the access logs generated by the HttpStack class.  I've tested by building a Homestead image with the change, where an example of the output is as follows.

22-05-2014 09:57:02.067 200 GET /impi/2010021254%40me2.cw-ngv.com/registration-status 0.000027 seconds

(I also fixed up a minor coding standards violation - a member variable was missing the _ prefix, which confused me while I was trying to work out the change.)

I'd raised cpp-common issue #102 to cover this.

I'll pull into sprout, homestead and ralf later as I suspect I am going to have some memcachedstore fixes as well.

Mike