refactor: leader schedule generation

[jstarry]

Problem

Need to do a little refactoring before modifying the leader schedule in a follow up change (#4597)

Summary of Changes

Refactored leader schedule generation code so that parts can be shared with the new vote account keyed leader schedule algorithm

Fixes #

[AshwinSekar]

lgtm just some minor nits

[AshwinSekar]

nit: you could compare the tuples directly:

(r_stake, r_pubkey).cmp(&(l_stake, l_pubkey))

[jstarry]

This PR just moves this sort_stakes function from another file, I'm fine with leaving it as is

[AshwinSekar]

nit: you could avoid the intermediate collection here by taking in an iter (flipped) Iterator<Item = (u64, &Pubkey)> and using the iter variants in sort_stakes:

stakes.sorted_unstable().dedup().reverse()

[jstarry]

IterTools often allocates internally in its helper methods FYI:

fn sorted_unstable(..) {
        let mut v = Vec::from_iter(self);
        v.sort_unstable();
        v.into_iter()
}