protocol/patricia: use sha3pool
protocol/patricia: use sha3pool

According to a profile, patricia.hashChildren accounts for ~2% of all
allocs:

```
  44955019  1.97% 54.85%   88884700  3.90%
chain/protocol/patricia.hashChildren
```

```
benchmark              old ns/op     new ns/op     delta
BenchmarkInserts-4     188643819     132184221     -29.93%

benchmark              old allocs     new allocs     delta
BenchmarkInserts-4     678692         283499         -58.23%

benchmark              old bytes     new bytes     delta
BenchmarkInserts-4     140052936     17648297      -87.40%
```

Closes chain/chainprv#2076

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain