[PATCH 00/11] Add ghost messages and fix thread linking

[Austin Clements]

This series modifies our database representation of messages that have
been referenced by other messages, but for which we don't have the
message itself.  Currently, we store this information as Xapian
metadata, but this has several downsides for performance and
complexity and results in hard-to-fix thread linking bugs.  This patch
series implements "ghost messages", which replace this Xapian metadata
with Xapian documents that look and act very much like regular message
documents, but simply have no content.  This simplifies and speeds up
our thread linking algorithm and fixes the currently broken thread
linking test.

Ghost messages also open up interesting future possibilities, such as
"pre-seeding" tags for messages that are not yet indexed.  This could
be used to make notmuch insert simpler and more robust, as part of tag
synchronization, and to improve nmbug's behavior when tags arrive
before messages.