Cache: Add ReadThrough mode

[bartelink]

This makes decider.Query(..., load = Equinox.AllowStale (TimeSpan.FromSeconds 1) opt into:

1. have all reads where the cache entry is stale or missing share a single roundtrip to load the state from the store
2. cache the value for the specified time (from the start of the retrieval, not when it arrived)

This guarantees that the effects of processing reads against a stream whose core function is to manage the write side are limited

• the readers within a given process will self-limit themselves to maximum one roundtrip per specified period
• if a writer updates in the interim (using the same Cache), ReadThrough reads can reuse that value, avoiding a retrieval roundtrip

Open questions:

• Should Cache implement IDisposable (it will trigger warnings in client mode as Cache construction will have to be called via new Cache) No, people don't Dispose Caches IRL is the position for now...
• Should caching of state be made possible for MemoryStore too? Nah, not this time.
• Should AllowStale become AnyCachedVersion given the fact that less surprising semantics are now available? (should be considered alongside naming of MaxStaleness)
• Rename MaxStaleness to AllowStale (means compiler errors for people leaning on the older behavior that's now entitled AnyCachedValue, which I believe is a good tradeoff on balance)

See also #388 and #380 for prep/tidy work on the way to this
Resolves #381

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@DSilence The ICache interface changes significantly, and is obviously harder to implement now, as it does more.
I don't hate it, but have to say I am tempted to remove ICache as I'm not convinced about there being meaningful implementations out there

Though it also has to be said that these series of PRs (esp the cleanup ones I linked above) are the realisation of a lot of stuff you were pushing for at the time you introduced them (recall I didn't want to have a common cache impl until it REALLY hurt and then figure out the best design at that point)

Spiked in #389

[nordfjord]

🙏

[bartelink]

BTW not certain whether there's value in separated metrics for incremental loads due to caching vs AllowStale roundtrips saved, or whether a cache hit is the main point - will be pondering