fix: u512 on reward computation (#10909)

Fix overflow found at
https://near.zulipchat.com/#narrow/stream/422293-core.2Fstake-wars-iv/topic/StatelessNet.20v84.20issue/near/430321806.

It happened with values
```
epoch_validator_reward = 1303530277894885217220048460336356
uptime_numer = 5149480000
stake = 19417599724822422137067484572677335
```
and `log2(epoch_validator_reward * uptime_numer * stake) is ~= 256.17`,
so it couldn't fit in `U256`.

However, it can fit in U512, because epoch_validator_reward and stake
are Balances = u128; uptime_numer fits in u64.

Note that final conversion to u128 is valid because
uptime_numer/uptime_denom and stake/total_stake are <= 1.

Co-authored-by: Longarithm <the.aleksandr.logunov@gmail.com>

chain/epoch-manager/src/reward_calculator.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 
 use num_rational::Rational32;
-use primitive_types::U256;
+use primitive_types::{U256, U512};
 
 use near_chain_configs::GenesisConfig;
 use near_primitives::checked_feature;
@@ -138,9 +138,9 @@ impl RewardCalculator {
                 // Apply min between 1. and computed uptime.
                 uptime_numer =
                     if uptime_numer > uptime_denum { uptime_denum } else { uptime_numer };
-                (U256::from(epoch_validator_reward) * uptime_numer * U256::from(stake)
-                    / uptime_denum
-                    / U256::from(total_stake))
+                (U512::from(epoch_validator_reward) * U512::from(uptime_numer) * U512::from(stake)
+                    / U512::from(uptime_denum)
+                    / U512::from(total_stake))
                 .as_u128()
             };
             res.insert(account_id, reward);