delete unnecessary real_users_cache, fix overwriting push_target iter, add proper function for getting local active users in room

this `real_users_cache` cache seems weird, and i have no idea what
prompted its creation upstream. perhaps they did this because
sqlite was very slow and their rocksdb setup is very poor, so
a "solution" was to stick member counts in memory.
slow iterators, scanning, etc do not apply to conduwuit where
our rocksdb is extremely tuned, and i seriously doubt something
like this would have any real world net-positive performance impact.

also for some reason, there is suspicious logic where we
overwrite the entire push target collection.

both of these things could be a potential cause for receiving
notifications in rooms we've left.

Signed-off-by: strawberry <strawberry@puppygock.gay>

src/service/sending/sender.rs

@@ -485,7 +485,7 @@ async fn send_events_dest_push(
 						.ok_or_else(|| {
 							(
 								dest.clone(),
-								Error::bad_database("[Push] Event in servernamevent_datas not found in db."),
+								Error::bad_database("[Push] Event in servernameevent_data not found in db."),
 							)
 						})?,
 				);
@@ -567,7 +567,7 @@ async fn send_events_dest_normal(
 							);
 							(
 								dest.clone(),
-								Error::bad_database("[Normal] Event in servernamevent_datas not found in db."),
+								Error::bad_database("[Normal] Event in servernameevent_data not found in db."),
 							)
 						})?,
 				);