[BUG] do not dequeue requests on a dead server

[BUG] do not dequeue requests on a dead server

Kai Krueger reported a problem when a server goes down with active
connections. A lot of connections were drained by that server. Kai
did an amazing job at tracking this bug down to the dequeuing
mechanism which forgets to check the server state before allowing
a request to be sent to a server.

The problem occurs more often with long requests, which have a chance
to complete after the server is completely marked down, and to find
requests in the global queue which have not yet been fetched by other
servers.

The fix consists in ensuring that a server is up before sending it
any new request from the queue.