BUG/MINOR: cli: dequeue from the proxy when changing a maxconn

BUG/MINOR: cli: dequeue from the proxy when changing a maxconn

The "set maxconn frontend" statement on the CLI tries to dequeue possibly
pending requests, but due to a copy-paste error, they're dequeued on the
CLI's frontend instead of the one being changed.

The impact is very minor as it only means that possibly pending connections
will still have to wait for a previous one to complete before being accepted
when a limit is raised.

This fix has to be backported to 1.6 and 1.5.