Option for changing maximum number of concurrent network requests

[soredake]

Is your feature request related to a problem? Please describe.
It would be nice to have an option for changing maximum number of concurrent network requests to have fewer timeouts on sites behind cloudflare #211

Describe the solution you'd like
Having such option.

Additional context
#211

[PatrickJRed]

+1
I agree.
I have 100k+ bookmarks so haveing all of them scanned with some delay (due to max concurrent net-conns) would make so BO doesnt get error due to server timeout or ddos/captcha responses