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fb4ba2bb - Speed up cold cache urllib3/boto3/botocore with batched prefetching (#2452)

Speed up cold cache urllib3/boto3/botocore with batched prefetching (#2452) With pubgrub being fast for complex ranges, we can now compute the next n candidates without taking a performance hit. This speeds up cold cache `urllib3<1.25.4` `boto3` from maybe 40s - 50s to ~2s. See docstrings for details on the heuristics. **Before**  **After**  --- We need two parts of the prefetching, first looking for compatible version and then falling back to flat next versions. After we selected a boto3 version, there is only one compatible botocore version remaining, so when won't find other compatible candidates for prefetching. We see this as a pattern where we only prefetch boto3 (stack bars), but not botocore (sequential requests between the stacked bars).  The risk is that we're completely wrong with the guess and cause a lot of useless network requests. I think this is acceptable since this mechanism only triggers when we're already on the bad path and we should simply have fetched all versions after some seconds (assuming a fast index like pypi). --- It would be even better if the pubgrub state was copy-on-write so we could simulate more progress than we actually have; currently we're guessing what the next version is which could be completely wrong, but i think this is still a valuable heuristic. Fixes #170.