I switched narrow.by_subject and narrow.by_recipient to use the all_msg_list
instead of current_msg_list, since we wanted to be able to narrow to messages
specifically not in the current_msg_list. However, in searches which revealed
old messages outside the range of all_msg_list (which only has a single contiguous range),
this broke narrowing.
Let's use msg_metadata_cache instead.
(imported from commit 427f717484b4ae83d9bb4cc6e51ce17177d037fe)