Implement SSH authentication support in Ruby

Closes #95

See merge request !154

Optimize gitlab-projects by deferring the loading of gitlab_reference_counter

See merge request !149

Bump gitaly-proto version to 0.20.0

See merge request !152

Support unhiding of all refs for Geo Nodes

Closes gitlab-ee#2959

See merge request !150

By default, we hide certain refs (e.g. keep-around/XXX) to save user
download time. We need to preserve these references on secondary Geo
nodes to have a true copy.

Loading gitlab_reference_counter loads gitlab_net, which adds an
additional 100-200 ms of time. That file pulls in a vendored Redis
library among other things.

Update vendoring for Gitaly

See merge request !148

Handle internal API errors when getting merge request URLs

See merge request !145

Update CHANGELOG

See merge request !144

Gitaly SSH Client

See merge request !139

Revert "Merge branch 'sh-disable-rubygems' into 'master'"

See merge request !143