Previously, setting esp_extra['x-smtpapi']['filters']
would override the entire filters setting, potentially
undoing other Anymail options that use SendGrid
filters (like track_opens).
Now, 'filters' is special-cased, and merged with
any other Anymail filter options.
(We don't do a fully deep merge, because otherwise
there would be no way to use esp_extra to *clear*
Anymail settings.)
* Add smtp-id in unique_args (metadata), to ensure
it shows up in click and open events.
* Add SENDGRID_GENERATE_MESSAGE_ID setting,
default True, to control auto-Message-ID behavior.
* Document it.
