BUG/MAJOR: server: prevent deadlock when using 'set maxconn server'

BUG/MAJOR: server: prevent deadlock when using 'set maxconn server'

A deadlock is possible with 'set maxconn server' command, if there is
pending connection ready to be dequeued. This is caused by the locking
of server spinlock in both cli_parse_set_maxconn_server and
process_srv_queue.

Fix this by reducing the scope of the server lock into
server_parse_maxconn_change_request. If connection are dequeued, the
lock is taken a second time. This can be seen as suboptimal but as it
happens only during 'set maxconn server' it can be considered as
tolerable.

This issue was reported on the mailing list, for the 1.8.x branch.
It must be backported up to the 1.8.