Update block hash computation

[benesjan]

Look for TODO(#3941) in the codebase.

Note: When I was logging calls to computeBlockHash in 1 test the function was called with the same value 3 times. Make sure this value gets stored on the Header object as it is expensive to compute.

[LHerskind]

This should be the hash of the header. See https://github.com/ethereum/go-ethereum/blob/f55a10b64d511b27beb02ff4978a6ed66d604cd8/core/types/block.go#L114 for geth code.