Rethink valid_since() and forget_before() API about time-steward

[elidupree]

I previously used the term settled for times when the user promised to submit no more fiat events – that was specifically for knowing when to compare checksums, but it could be the general term as well. So then we could be looking at

fn settle_before (&self, Time);
fn settled_before (&self)->Time;
fn forget_before (&self, ExtendedTime);
fn forgotten_before (&self)->ExtendedTime;

or maybe

fn settle_before (&self, Time);
fn earliest_unsettled_time (&self)->Time;
fn forget_before (&self, ExtendedTime);
fn earliest_remembered_time (&self)->ExtendedTime;

[elidupree]

Further complication: updated_until_before() could also be related to this. Internal code might want a way to refer to times that are not only settled from a caller perspective, but also no longer being mutated internally, which I think is generally equivalent to min(settled_before, updated_until_before)

[elidupree]

The word "freeze" is sometimes used for when data that used to be mutable is made immutable. That seems like a fitting concept for this, and clearer than "settle". Thus:

fn freeze_before(&self, Time);
fn earliest_mutable_time(&self)->Option<Time>;
fn forget_before(&self, Time);
fn earliest_remembered_time(&self)->Option<Time>;
fn latest_time_ready_for_snapshot(&self)->Option<Time>;