MINOR: server: create srv_was_usable() from srv_is_usable() and use a pointer

We used to call srv_is_usable() with either the current state and weights
or the previous ones. This causes trouble for future changes, so let's first
split it in two variants :
  - srv_is_usable(srv) considers the current status
  - srv_was_usable(srv) considers the previous status
9 files changed