Implement multipart form rewriting to support upload offloading

1. This parses multipart form data and saves all found files as files in TempPath. TempPath is received from Rails by calling authorize request. The rewritten multipart form data contains `file.path` where the temporary file is stored, and `file.name` the original name of file as stored in Content-Disposition. The temporary file is removed afterwards, if it's not consumed by GitLab Rails. If the body is not multipart, forward request.

2. All artifacts downloads are offloaded by exposing X-Sendfile-Type extension.


See merge request !5

Change identifying header to "Gitlab-Workhorse"



See merge request !8

Use httptest.Server instead of go run to spawn a new process

I don't like solution with `go run`. What do you think about this one?


See merge request !9

LFS storage api support

Part of gitlab-org/gitlab-ce#2955

See merge request !3

Refactor middleware

- Use gitRequest to store all processed request data
- Drop some headers when creating upstream request

@jacobvosmaer @marin: I still need to write test for it.


See merge request !7

- Use gitRequest to store all processed request data
- Drop some headers when creating upstream request

Serve GitLab uploads via gitlab-workhorse / X-Sendfile

Building on Kamil's work in https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/5

The xSendFile middleware should be reusable for other types of requests too (artifacts, LFS etc.).

See merge request !4

This is based on Kamil's work in
https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/5 .

Replace "auth request" with middleware

This gives us the flexibility we need to also handle regular downloads
and uploads in gitlab-workhorse.

See merge request !6

This gives us the flexibility we need to also handle regular
downloads and uploads in gitlab-workhorse.