Add /ngx_pagespeed_statistics

This adds /ngx_pagespeed_statistics. It also uses SharedMemoryStatistics instead of the SimpleStatistics currently used, and sets them as the default. It also adds support for tracking per server{} statistics.

Cleaning up of shared segments is disabled, because the current psol version we link in has a problem with the ordering of nginx's events during a configuration reload.

The referrer statistics will be ported in a separate pull.

Related to issue https://github.com/pagespeed/ngx_pagespeed/issues/133