BUG/MEDIUM: zero-weight servers must not dequeue requests from the backend

BUG/MEDIUM: zero-weight servers must not dequeue requests from the backend

It was reported that a server configured with a zero weight would
sometimes still take connections from the backend queue. This issue is
real, it happens this way :
  1) the disabled server accepts a request with a cookie
  2) many cookie-less requests accumulate in the backend queue
  3) when the disabled server completes its request, it checks its own
     queue and the backend's queue
  4) the server takes a pending request from the backend queue and
     processes it. In response, the server's cookie is assigned to
     the client, which ensures that some requests will continue to
     be served by this server, leading back to point 1 above.

The fix consists in preventing a zero-weight server from dequeuing pending
requests from the backend. Making use of srv_is_usable() in such tests makes
the tests more robust against future changes.

This fix must be backported to 1.4 and 1.3.