to support batch publishing messages
Replaces #602.
Batch publishing lets the system know there are multiple related
messages to be published so it can prioritize sending different messages
before sending copies of messages. For example, with the default API,
when you publish two messages A and B, under the hood A gets sent to D=8
peers first, before B gets sent out. With this MessageBatch api we can
now send one copy of A _and then_ one copy of B before sending multiple
copies.
When a node has bandwidth constraints relative to the messages it is
publishing this improves dissemination time.
For more context see this post:
https://ethresear.ch/t/improving-das-performance-with-gossipsub-batch-publishing/21713
