Fix CachingHttpContentFactory$CachedHttpContent already released buffer

[lorban]

Atomically retain the buffer with a check that it still is in the cache, otherwise delegate to the wrapped content.

Fixes #12681

[gregw]

@lorban also is there a version of this for 12.0?

[gregw]

I merged this to #12678 to check it works with the demos. All good there.

[lorban]

@gregw I backported this fix to 12.0.x here: #12710

The logic is sightly different as 12.0 does not have a writeTo() method but exposes retain() instead. It's quite ugly but since it's 12.0 only I think it's alright.

Merging both PRs to 12.1 is going to be fun!

[lorban]

@sbordet @gregw please come to an agreement about the private cachedRetain() method's name.

[gregw]

@sbordet @gregw please come to an agreement about the private cachedRetain() method's name.

I agree that cachedRetain() is not good. But I do think retainIfCached is a much clearer name than just retain. We have many retain methods and none of them do logic like this method, nor return a boolean.
Another alternative could be tryRetain