silly hack to allow migrating from conduit 0.9.0 to grapevine

Starting once with this patch will roll the db version back to 13, and
then grapevine can be run without the patch from that point onward.

The real way we want to do this is to finish the db compatibility break
work in !85, but this can serve as a stopgap.

src/database.rs

@@ -997,6 +997,14 @@ impl KeyValueDatabase {
                 Ok(())
             })?;
 
+            // Hack to ignore conduit's 13->16 migration. This is fine because
+            // the migration is idempotent, so it will just be run a second time
+            // as a no-op when switching from grapevine to conduit.
+            if services().globals.database_version().unwrap() == 16 {
+                services().globals.bump_database_version(13)?;
+                warn!("Reverted: 16 -> 13");
+            }
+
             assert_eq!(
                 services().globals.database_version().unwrap(),
                 latest_database_version,