Replaces `startVerifyUpstream` and behaves just like
`blocking.loading.strategy`.
We use the bootstrap resolver for any requests that arrive before the
upstreams are initialized.
Move `startVerifyUpstream` to `upstreams.startVerify` so it's accessible
via `UpstreamGroup` and we don't need to pass `startVerify` to all
resolver constructors that call `NewUpstreamResolver`.
Also has the nice benefit of greatly reducing the usage of `GetConfig`.
- `CacheControl.FlushCaches`
- `Querier.Query`
- `Resolver.Resolve`
Besides all the API churn, this leads to `ParallelBestResolver`,
`StrictResolver` and `UpstreamResolver` simplification: timeouts only
need to be setup in one place, `UpstreamResolver`.
We also benefit from using HTTP request contexts, so if the client
closes the connection we stop processing on our side.
ThinkChaos