Add date-related tracing headers

While doing email logging and archival for our platform, I've come up with a feature suggestion: To add optional date-related tracking headers:

• X-***-Enqueued-Date with the datetime when the mail has been generated or put in the queue.
• X-***-Sent-Date with the datetime the message is processed by a worker process.

This would add traceability to mesaging times and throughtputs. Dates should be RFC-2822 compliant (There is support to RFC-2822 format in PHP).

The '****' in the header names prefixing should be something appropiate, such as 'Drupal-Queue-Mail' or such. Th 'X-' prefix is strongly recommended for custom headers as per RFCs.