SendGrid: change message_id from Message-ID/smtp-id to UUID anymail_id

SendGrid does not always correctly provide the sent Message-ID header value 
to a tracking webhook's smtp-id field, making it unreliable to use for Anymail's 
`message_id`.

Instead, generate a UUID `message_id` for Anymail tracking, and pass it from 
send to webhooks in SendGrid custom args as anymail_id.

Webhooks will fall back to smtp-id for compatibility with previously-sent 
messages that didn't have an anymail_id custom arg.

Fixes #108

tests/utils.py

@@ -4,6 +4,7 @@ import logging
 import os
 import re
 import sys
+import uuid
 import warnings
 from base64 import b64decode
 from contextlib import contextmanager
@@ -165,6 +166,14 @@ class AnymailTestMixin:
         second = rfc822_unfold(second)
         self.assertEqual(first, second, msg)
 
+    def assertUUIDIsValid(self, uuid_str, version=4):
+        """Assert the uuid_str evaluates to a valid UUID"""
+        try:
+            uuid.UUID(uuid_str, version=version)
+        except (ValueError, AttributeError, TypeError):
+            return False
+        return True
+
 
 # Backported from Python 3.4
 class _AssertLogsContext(object):