api: Add status-messages to repo-updater and GraphQL API

Created by: mrnugget

This is the first part of the implementation of #4120 as described in https://github.com/sourcegraph/sourcegraph/issues/4120#issuecomment-499911568. (The second part will include the web component, UI, and changelog entry)

• Adds a /clone-queue-status endpoint to gitserver that returns the current and maximum number of repositories currently being cloned in parallel
• Adds a /status-messages endpoint to repo-updater that returns an array of status messages. Currently only one type exists, CURRENTLYCLONING, and that is only returned if the number of currently cloning repositories, as returned by gitserver, is non-zero.
• Adds a statusMessages query to the graphqlbackend that allows querying for status messages for site admins

The second part of this implementation will be a second PR that includes the web component of this feature.

Note: this is the minimal-viable implementation of this feature, as described in the ticket, and returns only the number of currently-being-cloned repositories.

The big caveat is that this number is not usable to indicate progress, it indicates activity. We have to evaluate/test/discuss whether that is "minimal-viable" for site-admins. Since the number could potentially be stuck at 5 for hours at a time if the Sourcegraph instance has been configured to only clone 5 concurrently (that's the default). But that would at least be an indicator that something happen, it's just not a progress-indicator and we have to be clear about this.

(cc @christinaforney)

Test plan: go test