simplify/fix email handling

[syjer]

Currently the email queue has some shortcomings coming from a previous design that must be addressed:

• the retry mechanism don't seems to work
• a more than necessary complexity in handling the retry/old emails
• the checksum mechanism for avoiding to send duplicate email should consider a window of time
• the full email message should only be loaded at the last moment, currently we are loading too much

Proposed design:

• eliminate notificationManager.processNotSentEmail.
• add a error_count field for the retry so we can have a backoff policy.
• The query for selecting the ids of the mail to be sent will be something like select id from email_message where status in ('WAIT', 'RETRY') and request_ts < sysdate)
• in case of error when sending the email, we set the request_ts in the future and increment the error_count and set the status to RETRY. The amount of minutes added from now depend of the number of error_count. If error_count is over a certain amount, the email status is marked as 'ERROR'.