Use simple BTreeMap to store uiaa requests

some uiaa requests contain plaintext passwords which should never be
persisted to disk.

Currently there is no cleanup implemented (you have to restart conduit)

src/database.rs

@@ -250,8 +250,7 @@ impl Database {
             },
             uiaa: uiaa::Uiaa {
                 userdevicesessionid_uiaainfo: builder.open_tree("userdevicesessionid_uiaainfo")?,
-                userdevicesessionid_uiaarequest: builder
-                    .open_tree("userdevicesessionid_uiaarequest")?,
+                userdevicesessionid_uiaarequest: RwLock::new(BTreeMap::new()),
             },
             rooms: rooms::Rooms {
                 edus: rooms::RoomEdus {