Make messageIDs bytes instead of strings

[rklaehn]

Rationale:

• go-libp2p currently produces non-utf8 strings
• making this bytes allows more flexibility in creating message ids, e.g.
  • random bytes
  • hashes

This issue was discovered in https://github.com/libp2p/rust-libp2p/issues/1671 . There is a coresponding issue in go-ipfs-pubsub, https://github.com/libp2p/go-libp2p-pubsub/issues/361

Changing this should have no negative impact. Implementations that expect valid utf-8 strings are already broken, because go-ipfs emits non utf8 strings. See https://github.com/libp2p/go-libp2p-pubsub/issues/361#issuecomment-663569492