Track cache hits and misses in accesslog

Currently doing work on an alternative DB-based cache backend for Drupal that increases cache hit rate, but finding it hard to get any reliable metrics on it. Would be handy to keep track of this in accesslog.

Maybe a -1 meaning "not intended to be cached" (e.g. logged in user or page cache is disabled), 0 for cache miss, 1 for cache hit.

Needs an extra column in accesslog for "cachehit," and an extra line in statistics_exit(). Is the X-Drupal-Cache header reliable enough to use for this?

Would also be lovely to backport to 7.x.