This commit adds a list new table, installations, which is used to keep
track of which installation are active for a given identity key.
In general, we limit the number of installation that we keep
synchronized to 5, to avoid excessive usage of resources.
Any installation coming from our own identity, will have to be manually
enabled, otherwise we trust the other peer has correctly paired their
devices.
We use a timestamp to decide which installations to keep synchronized as
a logical clock would have make the creation of the bundle more
complicated, but this can always be converted to a logical clock at
later stages without breaking compatibility.
Andrea Maria Piana