Signed-off-by: fuling <fuling.lgz@alibaba-inc.com>

- Remove unrelated changes
- Refactor code out of the API module - that is already getting pretty crowded.
- Don't track reference for AddFast in remote write.  This has the potential to consume unlimited server-side memory if a malicious client pushes a different label set for every series.  For now, its easier and safer to always use the 'slow' path.
- Return 400 on out of order samples.
- Use remote.DecodeWriteRequest in the remote write adapters.
- Put this behing the 'remote-write-server' feature flag
- Add some (very) basic docs.
- Used named return & add test for commit error propagation

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>