drop tombstones in client-server configurations

[tantaman]

• Brand new clients don't need to receive tombstones
• Server can drop tombstones once all clients it has ever seen are beyond a given version. This means it'll have to track last_seen for them which is currently not how it works.
• Clients can drop their tombstones as soon as they have been applied on the server

For the last case, we need some for of acknowledgement from the server to know that the changes were fully applied by them. We can loosely infer it if the server never resets our outbound stream but that certainly is not good enough. We can add periodic acks.

#38

#293