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/database/kvdatabase.rs

@@ -1,5 +1,5 @@
 use std::{
-	collections::{BTreeMap, HashMap, HashSet},
+	collections::{BTreeMap, HashMap},
 	path::Path,
 	sync::{Arc, Mutex, RwLock},
 };
@@ -150,7 +150,6 @@ pub struct KeyValueDatabase {
 	pub senderkey_pusher: Arc<dyn KvTree>,
 
 	pub auth_chain_cache: Mutex<LruCache<Vec<u64>, Arc<[u64]>>>,
-	pub our_real_users_cache: RwLock<HashMap<OwnedRoomId, Arc<HashSet<OwnedUserId>>>>,
 	pub appservice_in_room_cache: RwLock<HashMap<OwnedRoomId, HashMap<String, bool>>>,
 	pub lasttimelinecount_cache: Mutex<HashMap<OwnedRoomId, PduCount>>,
 }
@@ -265,7 +264,6 @@ impl KeyValueDatabase {
 			auth_chain_cache: Mutex::new(LruCache::new(
 				(f64::from(config.auth_chain_cache_capacity) * config.conduit_cache_capacity_modifier) as usize,
 			)),
-			our_real_users_cache: RwLock::new(HashMap::new()),
 			appservice_in_room_cache: RwLock::new(HashMap::new()),
 			lasttimelinecount_cache: Mutex::new(HashMap::new()),
 		})