Blueprints need to use a monotonic blueprint sequence for time

[jleibs]

Blueprints currently write updates into the blueprint using:

pub fn blueprint_timepoint_for_writes() -> TimePoint {
    TimePoint::from([(blueprint_timeline(), Time::now().into())])
}

We depend on the fact that this is monotonically increasing because we always do a latest-at query from the end of the timeline.

However, there are cases, for example saving a blueprint, setting back the system clock, and then re-loading the blueprint that can cause us to end up with items in the blueprint "from the future."

Our query semantics mean these items will always be returned and not overwritten since