Tracking Network modifications (serial ID)

[braginini]

What?
Introduce a serial (always incrementing ID) to the network that would increment on every network change (e.g. peer added/removed).
Why?
Support ordered network changes so that client apps could make sure they are applying recent changes avoiding unordered updates from the signal service.
How?
Add serial to the Account Network incrementing it on every change.
Should be an atomic operation.