ethereum/rpc/backend: (*EVMBackend).BlockBloom should create a global []byte slice for evmtypes.AttributeKeyEthereumBloom to reduce string->[]byte conversions

[odeke-em]

Noticed while auditing code and hunting for ways to improve transaction throughput, instead of incurring this continual expense in
https://github.com/tharsis/ethermint/blob/b8ae5984c54f79d76cfc95518f77b4226a05b940/ethereum/rpc/backend/backend.go#L202

you could just create a global like this

var bAttributeKeyEthereumBloom = []byte(evmtypes.AttributeKeyEthereumBloom)

and then later on

if bytes.Equal(attr.Key, bAttributeKeyEthereumBloom) {
     return ethtypes.BytesToBloom(attr.Value), nil
}

Only problem is that setting up a backend is so much work so no benchmarks, otherwise you can definitely see the improvement.