MINOR: counters: rename last_change counter to last_state_change

MINOR: counters: rename last_change counter to last_state_change

Since proxy and server struct already have an internal last_change
variable and we cannot merge it with the shared counter one, let's
rename the last_change counter to be more specific and prevent the
mixup between the two.

last_change counter is renamed to last_state_change, and unlike the
internal last_change, this one is a shared counter so it is expected
to be updated by other processes in our back.

However, when updating last_state_change counter, we use the value
of the server/proxy last_change as reference value.