emacs: Fix search tagging races

emacs: Fix search tagging races

This fixes races in thread-local and global tagging in notmuch-search
(e.g., "+", "-", "a", "*", etc.).  Previously, these would modify tags
of new messages that arrived after the search.  Now they only operate
on the messages that were in the threads when the search was
performed.  This prevents surprises like archiving messages that
arrived in a thread after the search results were shown.

This eliminates `notmuch-search-find-thread-id-region(-search)'
because these functions strongly encouraged racy usage.

This fixes the two broken tests added by the previous patch.