Add badger index cache

[SungJin1212]

Introduce a badgerDB (https://github.com/dgraph-io/badger) as a disk index cache. It could be a middle layer cache between in-memory and remote cache (in-memory -> (badger) -> Memcached or Redis).

There are two loop in the badger index cache, retentionLoop and diskStatUpdateLoop. retentionLoop: periodically run badgerDB GC to sink disk space. diskStatUpdateLoop: exports metric badger_value_log_space_available_bytes and badger_value_log_space_available_bytes track amount of disk space left on the value and key log used in Badger at mount point in bytes.

Which issue(s) this PR fixes: Fixes #6241

Checklist

• [X] Tests updated
• [ ] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[SungJin1212]

@yeya24 Using Size, we can track bytes usage of vlog, lsm. But, I should do more digging to delete keys when some threshold reached.

[alanprot]

This is a very interesting PR.. at some point i was thinking on doing something similar.

@SungJin1212 were you able to do some load test on this to see how it performs compared with the other caching solutions?

[SungJin1212]

@alanprot Thank you for your interest in my work. I haven't done the load test yet, but I was thinking that the load tests are necessary. Which performance aspects are essential in the load test do you think?

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