Set correct value of X-Forwarded-For header in PreAuthorize request

Closes #82

See merge request !104

Version 1.2.1 changelog

See merge request !103

Add CI related requests metrics

This MR adds some metrics for CI related requests.

Related to gitlab-org/gitlab-ce/#23366, gitlab-com/infrastructure#583

See merge request !97

Prometheus metrics for multipart file extraction

See merge request !100

API improvements: buffering, metrics

Following discussion in https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/94

This closes a hole in our Unicorn response buffering. The responses should be small, hence buffering in-memory.

See merge request !102

Stop leaking error messages in 502 responses

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/72

See merge request !101

Add "make release" script

See merge request !99

Version 1.2.0

See merge request !98

Accelerate multipart file uploads for all requests

Companion to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5867

Extends the upload mechanism for CI artifacts to all multipart file uploads.

See merge request !58

Handle environments/:id/terminal.ws, providing access to terminal websockets

A GitLab environment may expose a terminal connection for out-of-band access. Workhorse is responsible for providing a websocket connection to the terminal if present. 

It authenticates the user and retrieves connection details from GitLab using the environments/:id/terminal.ws/authorize endpoint, and sets up a proxy to the terminal provider, converting from the remote's subprotocol to a common format.

Authentication is periodically re-done, and the connection will be broken if it fails, or if the connection details change in any way.

This MR includes support for the `channel.k8s.io` websocket subprotocol, allowing connections to be made to Kubernetes / OpenShift containers. 

Based on top of (some of) !72 

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/22864

See merge request !83

A GitLab environment may expose a terminal connection for out-of-band access.
Workhorse is responsible for providing a websocket connection to the terminal
if present.

It authenticates the user and retrieves connection details from GitLab using
the environments/:id/terminal.ws/authorize endpoint, and sets up a proxy to
the terminal provider, converting from the remote's subprotocol to a common
format.

Authentication is periodically re-done, and the connection will be broken if
it fails, or if the connection details change in any way.

Version 1.1.1

See merge request !96

Rate limit builds register

Previously we were rate-limiting `/api` and `/ci/api`. This makes the it more targeted to only limit the capacity given to `builds/register.json` as this seems to be a main culprit of our stability problems.

@jacobvosmaer-gitlab @stanhu @pcarranza What do you think?

See merge request !95

Put archive tests in their own file

Felt like an easy way to trim down main_test.go a little.

See merge request !93

Test for tags

This would make it visible if we used a lightweight tag by accident.

See merge request !92

Version 1.1.0

See merge request !91

Prometheus metrics

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/61

This adds two metrics to begin with: total requests served,
partitioned by method/code, and the number of currently active
requests.

Things to check:

- [x] are the names of the metrics OK
- [x] is the access mechanism (unauthenticated HTTP at /metrics ) OK
- [x] check licenses of new dependencies: a mix of MIT, Apache 2.0, and BSD 3-clause

Note to reviewers: only the first four files in the GitLab diff are
outside vendor/. No need to scroll through the other 87 files.

See merge request !86