Add support for ssh certificates

Closes gitlab-ce#34572

See merge request gitlab-org/gitlab-shell!207

Update gitaly-proto to 0.109.0

See merge request gitlab-org/gitlab-shell!216

This along with the code submitted to gitlab-ce in the
gitlab-org/gitlab-ce! MR implements SSH certificate
authentication. See the docs added to gitlab-ce for why and how to
enable this. This, along with that MR, closes
gitlab-org/gitlab-ce#3457

Implementation notes:

 - Because it's easy to do, and because an earlier nascent version of
   this would pass user-ID to gitlab-shell, that's now supported, even
   though the SSH certificate authentication uses username-USERNAME.

 - The astute reader will notice that not all the API calls in
   gitlab-ce's lib/api/internal.rb support a "username" argument, some
   only support "user_id".

   There's a few reasons for this:

     a) For this to be efficient, I am bending over backwards to avoid
        extra API calls when using SSH certificates.

        Therefore the /allowed API call will now return a "user id" to
        us if we're allowed to proceed further. This is then fed to
        existing APIs that would only be called after a successful
        call to /allowed.

     b) Not all of the git-shell codepaths go through
        /internal/allowed, or ever deal with a repository, e.g. the
        argument-less "Welcome to GitLab", and
        /internal/2fa_recovery_codes. These need to use
        /internal/discover to figure out details about the user, so
        support looking that up by username.

     c) Once we have the "user id", the GL_ID gets passed down to
        e.g. user-authored hooks. I don't want to have those all break
        by having to handle a third GL_ID mode of "username" in
        addition to the current "key id" and "user id".

Add script to update vendored go libs

See merge request gitlab-org/gitlab-shell!212

Refactor for re-usability for future MR's

See merge request gitlab-org/gitlab-shell!210

This was not posing an issue during CI, but was an issue locally.

Run go tests in CI with Go 1.9 and 1.10

See merge request gitlab-org/gitlab-shell!208

Resolve "NoMethodError for nil in pre-receive hook"

Closes #138

See merge request gitlab-org/gitlab-shell!206

Update LICENSE year to 2018

[ci skip]

See merge request gitlab-org/gitlab-shell!205

allow long strings to remain intact while parsing broadcast message

See merge request gitlab-org/gitlab-shell!202

Added fix for msg nil edge case. fixed comment wording

code review issues, bumped version and changelog entry

Fixed rebase issues

Moved strip out of the function

Fixes for code review comments

Removed trailing whitespaces

Display the username instead of fullname

Closes #131

See merge request gitlab-org/gitlab-shell!204

When running `ssh git@gitlab.example.com` we used to reply with the
full name. But it might actually be more useful to display the
username. This is unique and can help support identifying issues.

Closes #131

Setup security products.

See merge request gitlab-org/gitlab-shell!201

Add missing GitlabLogger#error method

Closes #130

See merge request gitlab-org/gitlab-shell!200

Flush log file after every write

See merge request gitlab-org/gitlab-shell!199

Add Gitaly git-upload-archive migrated command

See merge request gitlab-org/gitlab-shell!192

Switch to structured logging

See merge request gitlab-org/gitlab-shell!193