* Archive or close thread and then directly open next or next unread thread
in originating thread-index.
* Drop use of old deprecated non _st functions, use their new names. Will
require 0.24 or newer.
* No longer support pre-lastmod notmuches.
* Catch missing content errors for messages, not fail.
* Wait and block R/O database opening, not fail.
* Defer changed_thread events on thread-index while still loading.
* Do not update tags in thread-view when not ready.
* Configuration option thread_view.expand_flagged determines whether flagged
messages are expanded by default.
* Better handle poll thread. Make sure to clean up connections here and in send_message.
* Polling can be cancelled with C-c in main window.