Remove errors from set methods on RemoteCacheClient interface

[56quarters]

What this PR does:

This change does a few things:

• Removes the error from the set method on the RemoteCacheClient interface. The only error that can be returned was an error when the async queue used for set operations was full. All other errors were logged and a counter incremented. Even for this error, it was special-cased and nil was returned instead. Thus, the interface included an error return value that could never actually happen.
• Adds another method, SetMultiAsync to the interface for setting multiple key-value pairs at once. This was the only functionality that the remoteCache struct was providing beyond the cache clients it was wrapping. With this new method, there's no longer a reason to keep it around (will address in a follow-up PR).
• Uses relative TTLs for Memcached items. Memcached supports using a relative TTL (e.g. 300 for an item that expires after 5 minutes) or absolute TTL (a particular UNIX timestamp). We were converting time.Duration objects to an absolute TTL for no reason.

Which issue(s) this PR fixes:

Part of #452

Checklist

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

[56quarters]

The old method was actually using the wrong var here, setting value from the argument to SetAsync instead of buf being passed to the closure.

[dimitarvdimitrov]

SGTM. I don't mind the error returned from SetAsync, especially because adding it back later might break linters and it's only an error. But maybe we'll cross that bridge when we come to it.

Found two outdated comments

[56quarters]

SGTM. I don't mind the error returned from SetAsync, especially because adding it back later might break linters and it's only an error. But maybe we'll cross that bridge when we come to it.

I'm OK removing the errors from RemoteCacheClient because we don't actually use that anywhere in Mimir except RemoteIndexCache in the store-gateway. Even there, the errors are logged instead of being returned (but again, they couldn't actually ever happen with the current implementations).

Found two outdated comments

Fixed, thank you.

[dimitarvdimitrov]

I'm OK removing the errors from RemoteCacheClient because we don't actually use that anywhere in Mimir except RemoteIndexCache in the store-gateway

hopefully one day dskit becomes a real library, not just a sub-package of mimir :D