This fixes performance bar not working with AJAX requests.

Both LabKit and Rails were injecting duplicate X-Request-Id headers into
the response stream, which caused the frontend to be interpret the value
as a comma-separated value (e.g. LdEgVXsp1R4, LdEgVXsp1R4).

The new flow behaves as the following:

1. Workhorse injects X-Request-Id to the inbound HTTP request to Rails
2. Rails looks up this value in its RequestId middleware. Since the
value exists, Rails sets the X-Request-Id response header to this value.
3. Workhorse doesn't append another header in the response.

Distributed tracing should still work fine because Workhorse still
generates the correlation ID.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60111

Closes #183

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.

Expose three options to configure queueing limits:
- apiLimit
- apiQueueLimit
- apiQueueTimeout

Workhorse usually connects to Rails over a Unix socket. This makes it
impossible to accidentally follow redirects to another host. This
change applies the same strictness when connecting to Rails over TCP.