MINOR: counters: move last_change into counters struct

MINOR: counters: move last_change into counters struct

last_change was a member present in both proxy and server struct. It is
used as an age statistics to report the last update of the object.

Move last_change into fe_counters/be_counters. This is necessary to be
able to manipulate it through generic stat column and report it into
stats-file.

Note that there is a change for proxy structure with now 2 different
last_change values, on frontend and backend side. Special care was taken
to ensure that the value is initialized only on the proxy side. The
other value is set to 0 unless a listen proxy is instantiated. For the
moment, only backend counter is reported in stats. However, with now two
distinct values, stats could be extended to report it on both side.