BUG/MEDIUM: queues: Stricly respect maxconn for outgoing connections

BUG/MEDIUM: queues: Stricly respect maxconn for outgoing connections

The "served" field of struct server is used to know how many connections
are currently in use for a server. But served used to be incremented way
after the server was picked, so there were race conditions that could
lead more than maxconn connections to be allocated for one server. To
fix this, increment served way earlier, and make sure at the time that
it never goes past maxconn.
We now should never have more outgoing connections than set by maxconn.