Generate a sync message to send to the peer represented by inState
inState
The doc to generate messages about
The SyncState representing the peer we are talking to
An array of [newSyncState, syncMessage | null] where newSyncState should replace inState and syncMessage should be sent to the peer if it is not null. If syncMessage is null then we are up to date.
[newSyncState, syncMessage | null]
newSyncState
syncMessage
Generate a sync message to send to the peer represented by
inState