Make append of chunks to be performant
Currently when appending chunks over Redis we always read a chunk, add data, and write it.
This is super slow, as reflected on https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/4667#note_206627709.
We should rather make the Redis operation to truncate (up to offset), append, and confirm.
We don't really seem to need to read the chunk to perform append of 3-100 bytes.