Anti-replay requirements

[andyleiserson]

This is a TODO section in the spec and was also the subject of some discussion at TPAC.

[csharrison]

A few things I recall discussing at TPAC:

1. Grouping together the state associated with multiple reports via origin / timestamp for efficiency (we do this in ARA)
2. Adding some associated data in the AEAD that allows the API caller to further partition the groups in a finer-grained way
3. Whether to support requerying the same report via the stateful anti-replay protection (vs. just emitting multiple reports).

IMO I think (2) and (3) are probably puntable from level 1 since it seems fairly easy to extend in the future, but we should specify (1) at the minimum.