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, 2 insertions, 0 deletions

diff --git a/lib/database-private.h b/lib/database-private.h
index 643b0507..5891584e 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -33,6 +33,8 @@ struct _notmuch_database {
     Xapian::QueryParser *query_parser;
     Xapian::TermGenerator *term_gen;
     Xapian::ValueRangeProcessor *value_range_processor;
+
+    notmuch_bool_t needs_upgrade;
 };
 
 /* Convert tags from Xapian internal format to notmuch format.