Send an IMAP command, but don't wait for responses.

Send an IMAP command, but don't wait for responses.

	* providers/imap/camel-imap-command.c (camel_imap_command_start):
	Send an IMAP command, but don't wait for responses.
	(camel_imap_command_response): Read a single line of response from
	the server.
	(camel_imap_command, etc): Reimplement in terms of the new code.

	* providers/imap/camel-imap-folder.c (imap_rescan): Use
	camel_imap_command_start and camel_imap_command_response, and
	call camel_operation_progress after each line read from the
	server.
	(imap_update_summary): Likewise, although with more fudging on the
	precentages... Also, fix this so that if none of the new messages
	are cached, it only does a single FETCH, and if some of them are
	cached, it does two FETCHes (one to get the UIDs, FLAGS, and
	SIZEs, and another to get the bodies of all of the messages that
	aren't cached now that it knows the relevant UIDs). This should
	speed up startup a bunch (especially if you have high bandwidth
	but also high latency to the IMAP server).