Persist tokens in db

OAuth2 tokens for SMTP server connection need to be persisted in a database in a newly created table (smtp_oauth2_tokens) to share access for all nodes in a cluster. This is different compared to graph connections which store tokens in a sandbox file. Same properties as for the sandox token file should be persisted - access token, refresh token, expire time and connection signature (to check if smtp connection parameters changed after last (re)authorization and the connection needs re-authorization). There should be always only one record in the table representing the latest authorization/re-authorization.