mod_http_oauth2: Reference grant by id instead of value Fixes that the grant got mutated on use of refresh token, notably it would gain 'id' and 'jid' properties set there by mod_tokenauth. Previously also the secret token that we should not be remembering.

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

}
s2s_whitelist = {
    "example.org",
}
```