DefaultLeaderElector::elect_leader binary search

[matteojug]

In DefaultLeaderElector::elect_leader, the binary_search_by is not really helping since we are constructing the cumulative_weights array each time: either we build it once (and then use the bin search, but note that there are other places where we still do a linear scan of the weights), or we can just find the matching index in the for loop above (~leader.rs:139).

[NikitaMasych]

Yep, thank you. I modified election to deduce party index on the go of prefix sums; won't do caching of cumulative_weights since design of the LeaderElector shall not care about Party's state.