light-client: Optimize voting power calculation by breaking the loop when we have enough voting power

[i-m-aditya]

Fixes #235

• [x] Referenced an issue explaining the need for the change
• [ ] Updated all relevant documentation in docs
• [x] Updated all code comments where relevant
• [ ] Wrote tests
• [x] Added entry in .changelog/

[romac]

Thanks @i-m-aditya! 💐