Replace dgraph-io/badger cache storage with etcd-io/bbolt

[stefans-elastic]

Proposed commit message

Replacing dgraph-io/badger persistent storage for key-value cache with etcd-io/bbolt. Originally it was meant to just get rid of go.opencensus.io dependency which is introduced by badger (please see parent issue for more details). After it got evident that this won't erase go.opencensus.io dependency it was decided that this work still should be done since etcd-io/bbolt is already used elsewhere in the project and it isn't a good thing to have multiple storages for cache (again, please see parent issue for more details (in comments)).

Implementation should be fairly straight-forward but I would like to clarify one thing - since bolt doesn't support value expiration the expiration time (and TTL) are stored as metadata of the value. Upon value retrieval it is checked for expiration and if it is expired then nil is returned and value gets deleted from bolt DB.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Author's Checklist

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How to test this PR locally

Related issues

• Closes Drop dbadger-io dependency #42443

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