Exclude seal when calculating the block hash about codechain

[remagpie]

When someone sends block with invalid seal, it can be marked as bad while importing. If someone tries to send same block with correct seal, it will be blocked because bad blocks are recorded forever.
Maybe we need to differentiate blocks with invalid seal from "real" bad bloks.

[majecty]

I'll add an external_seal field to the header. When calculating a hash of a block, the external_seal field will be not included.

[majecty]

We concluded not to exclude seal when calculating the block hash.
To give a penalty to a sleeping node, we should share which validators have signed to a block. And the validator set should be fixed in a block.