deps/objstore: upgrade to minio-go/v7

[heimweh]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

First time contributor so might need some handholding here :)

Upgraded from minio-go/v6 to minio-go/v7 and adjusted a couple functions to resolve the following issue: #2097

Example timings before (compactor):

level=info ts=2020-08-03T06:35:40.584914398Z caller=fetcher.go:452 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=13.712880984s cached=61 returned=61 partial=0
level=info ts=2020-08-03T06:35:47.210427484Z caller=fetcher.go:452 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=6.624377812s cached=61 returned=61 partial=0
level=info ts=2020-08-03T06:35:54.223933248Z caller=fetcher.go:452 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=27.351742657s cached=61 returned=31 partial=0

Example timings after (compactor):

level=info ts=2020-08-03T06:47:50.632665389Z caller=fetcher.go:453 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=135.100236ms cached=61 returned=61 partial=0
level=info ts=2020-08-03T06:48:50.682027929Z caller=fetcher.go:453 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=184.352188ms cached=61 returned=61 partial=0
level=info ts=2020-08-03T06:49:50.590307937Z caller=fetcher.go:453 component=block.BaseFetcher msg="successfully synchronized block metadata" duration=92.617787ms cached=61 returned=61 partial=0

Fixes: #2097
Related: minio/minio-go#1321

Verification

Existing tests and from actual usage.
Happy to provide more information and/or add tests if necessary :)

[bwplotka]

This looks very solid, thanks. Only one nit, but so small I will just merge, let's fix next time. 💪

It looks like the 100th contribution not first!

I really tried to find any suggestion to make, but could not ;p

[bwplotka]

👍

[bwplotka]

This is used once, so we can inline (:

See: https://thanos.io/contributing/coding-style-guide.md/#avoid-defining-variables-used-only-once

[bwplotka]

Wow (: They did this finally