When sending messages in quick succession, it might be that multiple
messages are batched together in datasync, resulting in a single large
payload.
This commit changes the behavior so that we can pass a max-message-size
and we split the message in batches before sending.
A more elegant way would be to split at the transport layer (i.e
waku/whisper), but that would be incompatible with older client.
We can still do that eventually to support larger messages.
