Enable lazy_load_members filter

I think we can enable lazy_load_members, it should cause any troubles with server which don't implement it yet except that the first sync takes longer then on a up-to-date sever.

https://matrix.org/docs/spec/client_server/unstable.html#get-matrix-client-r0-sync Related to #362 (closed) but actually a different issue

@aledomu can you add the filter? It should be basically this patch (except that we use serde_json to parse the json)

diff --git a/fractal-matrix-api/src/backend/sync.rs b/fractal-matrix-api/src/backend/sync.rs
index d19284a..ad95689 100644
--- a/fractal-matrix-api/src/backend/sync.rs
+++ b/fractal-matrix-api/src/backend/sync.rs
@@ -39,9 +39,8 @@ pub fn sync(bk: &Backend, new_since: Option<String>, initial: bool) -> Result<()
         let filter = format!(r#"{{
             "room": {{
                 "state": {{
-                    "types": ["m.room.*"],
-                    "not_types": ["m.room.member"]
-                }},
+                    "lazy_load_members": true
+                    }},
                 "timeline": {{
                     "types": ["m.room.message", "m.sticker"],
                     "limit": {}