Previously we limited Discourse Connect provider to 1 secret per domain.
This made it pretty awkward to cycle secrets in environments where config
takes time to propagate
This change allows for the same domain to have multiple secrets
Also fixes internal implementation on DiscourseConnectProvider which was
not thread safe as it leaned on class variables to ferry data around
Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>
Co-authored-by: David Taylor <david@taylorhq.com>