New MsgBase method: get_index() - returns an array of all message index records

(objects, in the same format as returned by the get_mgs_index() method)
much faster than iterating through a loop, calling the get_msg_index() method
for each message. If you want to load messages (e.g. headers, text), filtering
by criteria found in the message index (attributes, to/from user, subject CRC)
loading a list of indexes and filtering before calling get_msg_header() for the
selected messages is much faster than previously available MsgBase object
methods (e.g. get_all_msg_headers()). If you don't need to filter the loaded
messages (you really want *all* message headers), then get_index() is of no
benefit to you, the script-writer. This is most useful for the "mail" msgbase
where selective loading of messages is more common.