Harden email code

[sbrossie]

Currently, when the plugin attempts to send an email, if this fails for any reason, the code just throws an EmailException. Instead we need to implement the retry mechanism.

• Integrate with the plugin retry mechanism. Configuration should be done via YAML configuration (like https://github.com/killbill/killbill-analytics-plugin#per-tenant).
• Ensure idempotency, so that if another plugin throws an exception, emails aren't duplicated (we need another table to keep track of emails sent).