Reduce memcached allocations

[joe-elliott]

What this PR does:
I discovered this opportunity while researching panics in the querier. This PR reduces allocations related to using memcached caching by pooling buffers.

• Use the memcached allocator option to pull buffers from a pool on read
• Return buffers to the pool in frontend and object storage cache
• Moved the prealloc buffer pool to pkg/pool to be used by cache
• Removed the logic to copy before setting in cache. This is synchronous in both memcached and redis so there is no reason to copy.
• Added a changelog for Bugfix? Improve EOF handling in readers.go #4963 b/c I forgot!

Checklist

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

[javiermolinar]

Nice clean up