Use epoch_height to generate schedule instead of last_id

I had suggested the last_id, but that puts an unnecessary dependency
on LastIdsQueue. Using epoch height is pretty interesting in that
given the same set of stakers, you simply increment the seed once
per epoch.

Also, tighten up the LeaderSchedule code.

runtime/src/leader_schedule.rs

@@ -12,27 +12,20 @@ pub struct LeaderSchedule {
 }
 
 impl LeaderSchedule {
-    pub fn new(ids_and_stakes: &[(Pubkey, u64)], seed: &[u8; 32], len: u64) -> Self {
-        let (pubkeys, stakes): (Vec<Pubkey>, Vec<u64>) = ids_and_stakes
-            .iter()
-            .map(|&(ref id, ref stake)| (id, stake))
-            .unzip();
-
-        // Should have no zero weighted stakes
-        let mut rng = ChaChaRng::from_seed(*seed);
+    // Note: passing in zero stakers will cause a panic.
+    pub fn new(ids_and_stakes: &[(Pubkey, u64)], seed: [u8; 32], len: u64) -> Self {
+        let (ids, stakes): (Vec<_>, Vec<_>) = ids_and_stakes.iter().cloned().unzip();
+        let rng = &mut ChaChaRng::from_seed(seed);
         let weighted_index = WeightedIndex::new(stakes).unwrap();
-        let slot_leaders = (0..len)
-            .map(|_| pubkeys[weighted_index.sample(&mut rng)])
-            .collect();
-
+        let slot_leaders = (0..len).map(|_| ids[weighted_index.sample(rng)]).collect();
         Self { slot_leaders }
     }
 
     pub fn new_with_bank(bank: &Bank) -> Self {
+        let id_and_stakes: Vec<_> = bank.staked_nodes().into_iter().collect();
         let mut seed = [0u8; 32];
-        seed.copy_from_slice(bank.last_id().as_ref());
-        let stakes: Vec<_> = bank.staked_nodes().into_iter().collect();
-        Self::new(&stakes, &seed, bank.slots_per_epoch())
+        seed[0..8].copy_from_slice(&bank.epoch_height().to_le_bytes());
+        Self::new(&id_and_stakes, seed, bank.slots_per_epoch())
     }
 }
 
@@ -73,8 +66,8 @@ mod tests {
         let mut seed_bytes = [0u8; 32];
         seed_bytes.copy_from_slice(seed.as_ref());
         let len = num_keys * 10;
-        let leader_schedule = LeaderSchedule::new(&stakes, &seed_bytes, len);
-        let leader_schedule2 = LeaderSchedule::new(&stakes, &seed_bytes, len);
+        let leader_schedule = LeaderSchedule::new(&stakes, seed_bytes, len);
+        let leader_schedule2 = LeaderSchedule::new(&stakes, seed_bytes, len);
         assert_eq!(leader_schedule.slot_leaders.len() as u64, len);
         // Check that the same schedule is reproducibly generated
         assert_eq!(leader_schedule, leader_schedule2);