Add support to change max concurrent connections per server save basis

For every server I should be able to store max connections,
as some HP switches just allow one connection only so I can't do background transfers for firmware update. But that's what I want with all other servers.
Now these settings are global and can't be overwritten by per saved session settings.

Also all settings should be defined global and only if I have a difference between global or if I want to force sth. It should be settable.

This request is being tracked already:
Issue 1502 – Per-site limit of maximal number of background transfers
You can vote for it there.