store,compact,sidecar: replace all usage of json.Decoder with json.Unmarshal

[tmhdgsn]

fixes issue #1160

Changes

replaced usages of json.NewDecoder Decode, with ioutil.ReadAll/File and json.Unmarshal

Verification

been running in production for a few days, memory graphs appear marginally better fixes

[GiedriusS]

It looks OK from the code perspective but need to check myself just how much it reduces RAM consumption. Will do that soon 😄 thanks for the PR!

[GiedriusS]

On my cluster the change is inconsequential:

The difference is only ~50MiB which you can attribute to the different queries which come into my cluster since they are load-balanced. My cluster has ~2TiB data so it's not a small one :(

What's the difference on your cluster with this change? Do you have any graphs?

[bwplotka]

The graph you shown @GiedriusS is for what component? And if store is while doing any requests? The benefit should be seen mostly during traffic to store gateway (:

However this definitely does not solve #448 IMO, only helps slightly.

IMO we should still merge this as json.Decoder should not be used when you don't need streaming indeed. LGTM and thanks for this @tmhdgsn !

[GiedriusS]

It is definitely answering to queries hence the sharp-tooth pattern that you see (: that is Thanos Store, by the way. Either way, technically it is a good change.