Allow messages to be marked as "don't store"

[jm-clius]

This forms the implementation part of vacp2p/rfc#441

Problem description (copied from: vacp2p/rfc#441)

Sometimes the application might produce messages that don't need to be stored, an example would be a ping to show that you are online, or for example messages that are time sensitive (say a push notification, through our current push notification system, which would be ignored if retrieved at a later date).

In those cases it would be useful to mark them as "don't store", so that they are not stored by store nodes. This would improve bandwidth performance of the overall network and of the clients, at the cost of some metadata leaking (though can be optional).