Persist subscriptions

[jeromegn]

These (rather hefty) changes make Corrosion persist subscriptions across restarts.

This enables a few optimizations such as:

• No need to re-query large datasets on boot, this could've caused major issues if there was a reboot loop and clients were reconnecting thundering herd-ly.
• Clients can track the last update they've seen and subscribe from that point on. This should prevent losing data. If a client detects a gap, they know they've missed a change.