timeline durability

[carlsverre]

Timelines should be durably stored on the client using #15 and also durably stored on the coordinator.

Need to think about what happens if the client loses its timeline state, but still knows its identity. Should it pull down pending mutations from the server, or should it create a new timeline id and act as a new client. Since timeline is durable, this would be correct - although to the client it may appear that data has been temporarily lost until the server incorporates it. On the other hand, this is a pretty rare edge case - and it makes sense that local durability should be treated as all or nothing.