Two new functions in the libnotmuch API:
`notmuch_message_count_files`, and `notmuch_thread_get_total_files`.
+New function to remove properties
+
+ A new function was added to the libnotmuch API to make it easier to
+ drop all properties with a common pattern:
+ `notmuch_message_remove_all_properties_with_prefix`
+
Change of return value of `notmuch_thread_get_authors`
In certain corner cases, `notmuch_thread_get_authors` previously