Rework the way the TimestampService works

[MaddieM4]

Instead of a services module, we can just have a timestampers module, and a TimestampService interface of:

type TimestampService interface {
    // Get an ordered list of timestamp IDs
    GetTimestamps(topic string) []string
}

If we just treat timestamps as strings, rather than objects, it makes implementations of TimestampService a lot more flexible, and completely obviates the need for TimestampManagers. It also allows us to abstract all the high-level validation and traversal logic into an external system, so that none of the Document or DocState internals need to care about it.

[MaddieM4]

Creating the TimestampTracker in another ticket.