MEDIUM: proxy: add and use a separate last_change variable for internal use

MEDIUM: proxy: add and use a separate last_change variable for internal use

Same motivation as previous commit, proxy last_change is "abused" because
it is used for 2 different purposes, one for stats, and the other one
for process-local internal use.

Let's add a separate proxy-only last_change variable for internal use,
and leave the last_change shared (and thread-grouped) counter for
statistics.