Gossipsub message id to shortened bytes (#1607)
Gossipsub message id to shortened bytes (#1607)

## Issue Addressed

https://github.com/ethereum/eth2.0-specs/pull/2044

## Proposed Changes

Shifts the gossipsub message id to use the first 8 bytes of the SHA256 hash of the gossipsub message data field.

## Additional Info

We should merge this in once the spec has been decided on. It will cause issues with gossipsub scoring and gossipsub propagation rates (as we won't receive IWANT) messages from clients that also haven't made this update. 




sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse