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.

Accomodate broken HTTP clients

HTTP header lookups are supposed to be case-insensitive but some
clients do a case insensitive lookup, which breaks because of Go's
header canonicalization.

See merge request !2