Connection: Add periodic test to let user know if connection broken

Currently, it is possible for a remote Jetpack site to have connection details store and for WP.com to not have a record of that.

At the moment, we don't surface this to users. We should find a way to test the connection periodically, perhaps when fetching the plan, and give the user some way to fix their connection.

The issue does not name files or tests. Start by locating the connection-detail storage and the plan-fetching path mentioned in the report, then trace how remote Jetpack and WP.com state are compared. Done means broken connections are checked periodically, surfaced to users, and given a recovery path.

Written by the indexing model from the issue text.