Optimize sort keys by server in memcache client

[yeya24]

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

Changes

When fetching a lot of keys from index cache using memcached, I see in our store gateway 13% CPU spent on MemcachedJumpHashSelector.PickServer. Among them most of the CPU spent on Each which is the function to get all the memcached server addresses.

For each key we call PickServer once and each PickServer call requires getting all memcached server addresses by calling Each function. https://github.com/thanos-io/thanos/blob/main/pkg/cacheutil/memcached_server_selector.go#L65

This is unnecessary because memcache server addresses update infrequently. When there are a lot of keys to fetch through memcached, it can be expensive to call Each every time.

This PR I try to cache server addresses in SetServers so that we don't have to call Each to get addresses every time. Instead we call Each once in the sortKeysByServer method to get all addresses and do jump hash using the fetched addresses to help reduce lock contention.

Verification

[harry671003]

Thanks. Change looks good overall.
Lint is failing.