Send multiple messages in one connection

[cha798215]

As Django document said, establishing and closing an SMTP connection (or any other network connection, for that matter) is an expensive process. So we should reuse an SMTP connection to send multiple emails, rather than creating and destroying a connection every single email.

This PR change the strategy of sending email. It is no longer sending a task for each email message one by one. It slice the email messages to many of chunks and send a task for each chunk. You can set your own DJCELERY_SES_CHUNK_SIZE to control how many email messages you want to send by every single connection.

Because of async tasks, it can only get ids of the tasks, instead of the real result of sending mails. So send_messages will return the length of the argument email_messages.

[Robert-Chiang]

Suggested change

	pages, remainder = divmod(len(email_messages), CHUNK_SIZE)
	if remainder != 0:
	pages += 1
	for page in range(pages):
	offset = CHUNK_SIZE * page
	email_messages_chunk = email_messages[offset: offset + CHUNK_SIZE]
	for index in range(0, len(email_messages), CHUNK_SIZE):
	email_messages_chunk = email_messages[index: index + CHUNK_SIZE]