Synchronize deletions

[ironfroggy]

For now, objects can only be added and updated. They cannot be deleted.

The problem is how to synchronize. The central server needs to know which things have been deleted to tell the clients, but doesn't know when clients might still be waiting for new delete messages.

• Keep a list of all deleted keys, forever?
• Send current keys when store sizes don't match?
• Keep a list of recent deletions + send a full key list when counts don't match?

Most likely will do Option 1 first, and later upgrade this to Option 3.