Client: add block validation to chain syncing

[holgerd77]

Description

Currently validation on chain syncing is not taking place, the blockchain class is initialized with validation set to false.

This should be changed. 😄

Possible Implementation

Work is largely taking place on the ethereumjs-block library, which has to be backported to support the pre-Byzantium validation rules, see PR below.

Depends on ethereumjs/ethereumjs-block#46 and ethereumjs/ethereumjs-blockchain#54.

[aunyks]

Is this resolved or close to resolution?

[vpulim]

Enabling the validate flag should pretty much work now for PoW chains. PoA chains like Rinkeby will still fail validation until these issues are solved:

https://github.com/ethereumjs/ethereumjs-block/issues/45
https://github.com/ethereumjs/ethereumjs-blockchain/issues/76

[holgerd77]

Fixed in #1075, will close.