This commit fixes a bug on the mvds library where the nextEpoch would be
incorrectly summed to the retry time, resulting in messages not being
retried, or retried much less frequently the longer the app was running.
It also updates the retry timing to backoff exponentially at multiple of
30 seconds.
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.
