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

Disable NGINX response buffering when not proxying

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/78

See merge request !87

Too many things on prometheus.gitlab.com are already called
'http_requests_total'.

Avoid setting multiple values for certain headers

Content-Type, Content-Disposition and other headers were being given multiple values in some workhorse responses, causes problems with, e.g., archive downloads.

This MR refactors the tests somewhat, and replaces calls to `Header().Add()` with `Header().Set()` where appropriate.

Closes #76

See merge request !84

Release v1.0.0

See merge request !82

Add "quick facts" section

Towards https://gitlab.com/gitlab-org/gitlab-workhorse/issues/75

See merge request !79

Adds Unix signal to reopen log file

This MR adds a command line flag to provide a log file and creates a handler to reopen the log file upon receiving a SIGHUP signal.

A new dependency is also added: [github.com/client9/reopen](https://github.com/client9/reopen)

Relevant Issue: [#52](https://gitlab.com/gitlab-org/gitlab-workhorse/issues/52)

cc: @jacobvosmaer-gitlab, @smcgivern 

See merge request !77

  - Adds new logFile flag
  - Adds client9/reopen dependency
  - Enables log file reopening on SIGHUP
  - Adds a custom response logger

Improve error messages

Less noise from 'deepen' inspection. Better grouping of 'bad gateway' errors.

See merge request !76

Hopefully this will lead to better grouping of errors in Sentry.

They are just noise.

Change the revspec for `git format-patch`

For merge requests, GitLab currently sends:

* ShaFrom == tip of target branch
* ShaTo == tip of source branch

This MR adds tests for this scenario and the alternative:

* ShaFrom == base SHA
* ShaTo == tip of source branch

It then makes the test for the first case pass by altering the revspec. 

What to do with diffs is still an open question, so we leave those untouched for the moment.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/22229

See merge request !68

Print go version during CI build



See merge request !74

Resolve "http.StatusTooManyRequests breaks Go 1.5 compatibility"

This merge request replaces a constant introduced in go1.6 with its literal value, as we need go1.5 support. No other changes are required.

It also modifies .gitlab-ci.yml to test against go1.5, 1.6 and 1.7, to prevent future regressions.

Closes #69

See merge request !73