lib: Implement versioning in the database and provide upgrade function.

The recent support for renames in the database is our first time
(since notmuch has had more than a single user) that we have a
database format change. To support smooth upgrades we now encode a
database format version number in the Xapian metadata.

Going forward notmuch will emit a warning if used to read from a
database with a newer version than it natively supports, and will
refuse to write to a database with a newer version.

The library also provides functions to query the database format
version:

	notmuch_database_get_version

to ask if notmuch wants a newer version than that:

	notmuch_database_needs_upgrade

and a function to actually perform that upgrade:

	notmuch_database_upgrade

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index 27b1317d..f9adea72 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -238,6 +238,9 @@ _notmuch_message_gen_terms (notmuch_message_t *message,
 			    const char *prefix_name,
 			    const char *text);
 
+void
+_notmuch_message_upgrade_filename_storage (notmuch_message_t *message);
+
 notmuch_status_t
 _notmuch_message_add_filename (notmuch_message_t *message,
 			       const char *filename);