Implement locktower voting (#3251)

* locktower components and tests

* integrate locktower into replay stage

* track locktower duration

* make sure threshold is checked after simulating the vote

* check vote lockouts using the VoteState program

* duplicate vote test

* epoch stakes

* disable impossible to verify tests

core/src/staking_utils.rs

@@ -58,7 +58,7 @@ fn node_staked_accounts(bank: &Bank) -> impl Iterator<Item = (Pubkey, u64, Accou
     })
 }
 
-fn node_staked_accounts_at_epoch(
+pub fn node_staked_accounts_at_epoch(
     bank: &Bank,
     epoch_height: u64,
 ) -> Option<impl Iterator<Item = (&Pubkey, u64, &Account)>> {