CLEANUP: server: reorder some fields in the server struct to respect cache lines

CLEANUP: server: reorder some fields in the server struct to respect cache lines

There's currently quite some thread contention in the server struct
because frequently fields accessed fields are mixed with those being
often written to by any thread. Let's split this a little bit to
separate a few areas:
  - pure config / admin / operating status (almost never changes)
  - idle and queuing (fast changes, done almost together)
  - LB (fast changes, not necessarily dependent on the above)
  - counters (fast changes, at a different instant again)