store: use k-way merging for proxying logic

Use k-way merging for merging multiple responses coming from multiple
StoreAPIs. This avoids having a bottleneck of a Go channel that can only
hold 10 responses at once. So, now we will Recv() messages as fast as
possible. Users should be able to see much quicker queries because
network will be used as much as possible.

Benchmarks:

```
name                                                                                     old time/op    new time/op    delta
ProxySeries/1000000SeriesWith1Samples/4_client_with_1_samples,_250000_series_each-16        1.01s ± 7%     0.24s ± 2%  -76.62%  (p=0.008 n=5+5)
ProxySeries/1000000SeriesWith1Samples/single_client_with_1_samples,_1000000_series-16       926ms ± 3%     454ms ± 2%  -50.99%  (p=0.008 n=5+5)
ProxySeries/100000SeriesWith100Samples/4_client_with_25_samples,_25000_series_each-16      95.6ms ± 1%    23.4ms ± 3%  -75.52%  (p=0.016 n=4+5)
ProxySeries/100000SeriesWith100Samples/single_client_with_100_samples,_100000_series-16    93.1ms ± 2%    45.1ms ± 1%  -51.63%  (p=0.008 n=5+5)
ProxySeries/1SeriesWith10000000Samples/4_client_with_2500000_samples,_1_series_each-16     39.5µs ± 1%    25.2µs ± 1%  -36.22%  (p=0.008 n=5+5)
ProxySeries/1SeriesWith10000000Samples/single_client_with_10000000_samples,_1_series-16    22.6µs ± 0%    15.2µs ± 0%  -32.71%  (p=0.008 n=5+5)

name                                                                                     old alloc/op   new alloc/op   delta
ProxySeries/1000000SeriesWith1Samples/4_client_with_1_samples,_250000_series_each-16        186MB ± 0%     108MB ± 0%  -41.87%  (p=0.008 n=5+5)
ProxySeries/1000000SeriesWith1Samples/single_client_with_1_samples,_1000000_series-16       186MB ± 0%     107MB ± 0%  -42.18%  (p=0.008 n=5+5)
ProxySeries/100000SeriesWith100Samples/4_client_with_25_samples,_25000_series_each-16      18.5MB ± 0%    10.0MB ± 0%  -45.79%  (p=0.008 n=5+5)
ProxySeries/100000SeriesWith100Samples/single_client_with_100_samples,_100000_series-16    18.5MB ± 0%    10.6MB ± 0%  -42.69%  (p=0.008 n=5+5)
ProxySeries/1SeriesWith10000000Samples/4_client_with_2500000_samples,_1_series_each-16     10.9kB ± 0%     6.1kB ± 0%  -44.06%  (p=0.008 n=5+5)
ProxySeries/1SeriesWith10000000Samples/single_client_with_10000000_samples,_1_series-16    5.09kB ± 0%    3.06kB ± 0%  -39.83%  (p=0.008 n=5+5)

name                                                                                     old allocs/op  new allocs/op  delta
ProxySeries/1000000SeriesWith1Samples/4_client_with_1_samples,_250000_series_each-16        5.00M ± 0%     1.00M ± 0%  -80.00%  (p=0.008 n=5+5)
ProxySeries/1000000SeriesWith1Samples/single_client_with_1_samples,_1000000_series-16       5.00M ± 0%     1.00M ± 0%  -80.00%  (p=0.008 n=5+5)
ProxySeries/100000SeriesWith100Samples/4_client_with_25_samples,_25000_series_each-16        500k ± 0%      100k ± 0%  -79.96%  (p=0.008 n=5+5)
ProxySeries/100000SeriesWith100Samples/single_client_with_100_samples,_100000_series-16      500k ± 0%      100k ± 0%  -79.98%  (p=0.016 n=5+4)
ProxySeries/1SeriesWith10000000Samples/4_client_with_2500000_samples,_1_series_each-16        180 ± 0%       125 ± 0%  -30.56%  (p=0.008 n=5+5)
ProxySeries/1SeriesWith10000000Samples/single_client_with_10000000_samples,_1_series-16       109 ± 0%        80 ± 0%  -26.61%  (p=0.008 n=5+5)
```

`TestProxyStore_SeriesSlowStores` also passes which tests how everything works with slow stores.

Rewritten chunk deduplication logic to use hashing.

In the future we could get some more marginal gains by using a selection/tournament tree.

Signed-off-by: Giedrius Statkevičius <[email protected]>

CHANGELOG.md

@@ -28,6 +28,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#5231](https://github.com/thanos-io/thanos/pull/5231) Tools: Bucket verify tool ignores blocks with deletion markers.
 - [#5244](https://github.com/thanos-io/thanos/pull/5244) Query: Promote negative offset and `@` modifier to stable features as per Prometheus [#10121](https://github.com/prometheus/prometheus/pull/10121).
 - [#5255](https://github.com/thanos-io/thanos/pull/5255) InfoAPI: Set store API unavailable when stores are not ready.
+- [#5255](https://github.com/thanos-io/thanos/pull/5296) Query: Use k-way merging for the proxying logic. This ensures that network is used as efficiently as possible. Also, the proxying sub-system now uses much less resources (~30-50% less CPU usage, ~30-40% less RAM usage according to our benchmarks). We intend to add a series limiter on Thanos Query in the near future as now Query buffers series responses as fast as possible.
 
 ### Removed