Add support for requesting out of order messages

[rabble]

As a user who tries to load a message we don't have, i should be able to request it from peers similar to how ssb-ooo works and there should be an updated UI saying that the message isn't on the device and we're looking at peers to get it.

Store it in a dir of individual messages Write to sqlite.

[danlatorre]

Design context: how do we convey the different states in this user flow to users?