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.