store: Add mixin to enable caching bucket

[kakkoyun]

This PR adds a mixin to enable caching bucket support.

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

Changes

• Add withCachingBucketMemcached for Thanos Store to enable caching bucket.
• Rename withMemcachedIndexCache to withIndexCacheMemcached for consistency.
• Bump version to the one include the feature.

Verification

• make generate
• Local test

[kakkoyun]

@pstibrany Could also have a look at it?

[pstibrany]

After merging thanos-io/thanos#2579, config has changed slightly (config_cache field has been removed, and more options are available):

backend: memcached
backend_config:
  addresses:
    - localhost:11211

chunk_subrange_size: 16000
max_chunks_get_range_requests: 3
chunk_object_size_ttl: 24h
chunk_subrange_ttl: 24h
blocks_iter_ttl: 5m
metafile_exists_ttl: 2h
metafile_doesnt_exist_ttl: 15m
metafile_content_ttl: 24h
metafile_max_size: 1MiB

Since that PR was merged without documentation change and CHANGELOG entry, I'm sending new PR shortly (working on it).

Update: thanos-io/thanos#2618

[pstibrany]

Heads up: another change incoming in PR thanos-io/thanos#2613 (chunk_object_size_ttl -> chunk_object_attrs_ttl)

[kakkoyun]

Marking it as WIP until Thanos v0.13 released, since upcoming release may include related changes.

[kakkoyun]

Thanos v0.13.0-rc.0 released, we can proceed with this https://github.com/thanos-io/thanos/releases/tag/v0.13.0-rc.0