Add prioritized validators. Filter prioritized validators when updating validator set.

[nxqbao]

Description

• Add prioritized validators
• Add functions to update priority threshold, prioritized validators
• Filter prioritized validators when updating the validator set: prioritized validators are picked first (up to the priority threshold); the remaining prioritized and non-prioritized ones are picked after
• Add test case

TODO

• Double check the case: the remaining prioritized and non-prioritized ones are picked by the highest-to-lowest voting power
• Measuring gas

Checklist

• I have clearly commented on all the main functions followed the NatSpec Format
• The box that allows repo maintainers to update this PR is checked
• I tested locally to make sure this feature/fix works

[phuctd95]

Replace setPrioritizedValidators function with add/remove functions. Each time, a validator is added to or removed from the whitelist.

The _arrangeValidatorCandidates function can be optimized. We only need 1 for loop. (We do not need to fix it now. I only put a note here, let's do this later).

[ducthotran2010]

The array _candidates is a memory variable, we may want to remove _arrangedValidators for gas saving.

[ducthotran2010]

I guess we can increase the weight for the whitelisted address (e.g adding to a fixed number)