Beaconblock over wire

[shayzluf]

[Part of] #3398

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We have requested headslot as start slot in the beacon block request added +1 to request the block ahead

We have previously sent and expected BeaconBlockRespone over the wire while it contained protobuf fields that other client don't send or expect to get. we have fixed the way we un marshal it from the wire

[codecov]

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[prestonvanloon]

Looking at this now, we can use interface{} instead of defining new methods specifically for blocks. Will push up shortly

[prestonvanloon]

LGTM.

More context:
https://github.com/ethereum/eth2.0-specs/blob/3d97160098ab2c6d494dc4ce4d1a9bcbb429df6d/specs/networking/p2p-interface.md#encoding-strategies

For objects containing a single field, only the field is SSZ-encoded not a container with a single field. For example, the BeaconBlocks response would be an SSZ-encoded list of BeaconBlocks.