EthRelay: Mixhash Verfiy Testing on Mainnet.

[hackfisher]

Because there is an issue related to Mixhash on Ropsten:

rust-ethereum/ethash#4

So the current Mixhash verification is disabled in Ropsten: #145

On mainnet it is only tested in unit tests, we may need to run a integration tests for relay_header calls.

[hackfisher]

Let's do a test on Crab Testnet.

That is, for crab testnet, there is no token bridge required for now (no ring backing like mainet etc)

But there are 1% of the RING tokens (20M RING) for backing all the cRINGs on Crab.

We can enable 100 cRING -> 1 RING redeem protocol for testing this issue

[hackfisher]

Related rust-ethereum/ethash#8