Add message.topic to gossipsub message-id in Altair

[ralexstokes]

Fixes #2471.

In lieu of discriminating duplicated sync committee messages as suggested in #2471 by adding the subnet_id to the message itself, this PR suggests a solution by incorporating the topic name in to the gossipsub message-id. This solution should allow sync committee message duplicates to still remain distinct in the gossipsub machinery without needing to add an additional field to the message type.

Per discussion w/ @djrtwo and @protolambda, this avoids a potential DoS vector w/ the additional (unsigned) field and should harden message propagation across fork boundaries (as the fork digest will enter via the message.topic). This solution should also be a bit less intrusive than changing the sync committee structures further. Obviously open to other alternatives discovered during implementation :)

[protolambda]

Hmm, I think we should clarify the transition process of the message ID. For altair it won't matter, but it has to happen all at the same time, or we get two different message IDs for the same message. With altair we drop all old topic names anyway, so I suppose that we just never use the new message ID function for old topics?