Specify eviction priority

[evanstade]

Currently, buckets can be marked as persistent or not, which affects eviction, but that's a binary and it may not be respected.
Buckets can have an expiration date, but that's also a binary state (either the date is in the future or the past).
Durability is actually about flushing data to disk, and not related to whether a bucket survives eviction passes. (also this is not currently part of the spec as it was culled from the MVP)

There's a gap here: sites should be able to specify an eviction priority for their data, e.g. so that their own log data is evicted before their own pinned documents, even if log data is more recently used[1] than pinned documents.

[1] this example assumes the user agent's internal eviction routine uses LRU as a heuristic