+Introduced database version 3 with support for "database features."
+
+ Features are independent aspects of the database schema.
+ Representing these independently of the database version number will
+ let us evolve the database format faster and more incrementally,
+ while maintaining better forwards and backwards compatibility.
+
+Library users are no longer required to call `notmuch_database_upgrade`
+
+ Previously, library users were required to call
+ `notmuch_database_needs_upgrade` and `notmuch_database_upgrade`
+ before using a writable database. Even the CLI didn't get this
+ right, and it is no longer required. Now, individual APIs may
+ return `NOTMUCH_STATUS_UPGRADE_REQUIRED` if the database format is
+ too out of date for that API.
+
+Library users can now abort an atomic section by closing the database
+
+ Previously there was no supported way to abort an atomic section.
+ Callers can now simply close the database, and any outstanding
+ atomic section will be aborted.
+