synchronize all cache mutations and supply sinceRelaySet so callers get snapshot copies

rework groupByRelayPresence to build relay snapshots via sinceRelaySet,
    filtering with immutable lists to prevent ConcurrentModificationException when relays update mid-iteration

amethyst/src/main/java/com/vitorpamplona/amethyst/service/relayClient/reqCommand/user/watchers/UserReportsSubAssembler.kt

@@ -90,19 +90,30 @@ class UserReportsSubAssembler(
         users: Iterable<User>,
         eoseCache: EOSEAccountFast<User>,
         inRelays: Set<NormalizedRelayUrl>,
-    ): Collection<List<User>> =
-        users
-            .groupBy {
-                eoseCache
-                    .since(it)
-                    ?.keys
-                    ?.intersect(inRelays)
-                    ?.hashCode()
+    ): Collection<List<User>> {
+        if (users.none()) return emptyList()
+
+        val relaySnapshot = inRelays.toSet()
+
+        return users
+            .groupBy { user ->
+                val relaysForUser = eoseCache.sinceRelaySet(user)
+                if (relaysForUser.isNullOrEmpty() || relaySnapshot.isEmpty()) {
+                    null
+                } else {
+                    val intersection = relaysForUser.filter { it in relaySnapshot }.sorted()
+                    if (intersection.isEmpty()) {
+                        null
+                    } else {
+                        intersection.hashCode()
+                    }
+                }
             }.values
             .map {
                 // important to keep in order otherwise the Relay thinks the filter has changed and we REQ again
                 it.sortedBy { it.pubkeyHex }
             }
+    }
 
     fun findMinimumEOSEsForUsers(
         users: List<User>,

amethyst/src/main/java/com/vitorpamplona/amethyst/service/relays/EOSE.kt

@@ -166,6 +166,11 @@ class EOSEAccountFast<T : Any>(
             users[key]?.relayList?.toMutableMap()
         }
 
+    fun sinceRelaySet(key: T): Set<NormalizedRelayUrl>? =
+        synchronized(lock) {
+            users[key]?.relayList?.keys?.toSet()
+        }
+
     fun newEose(
         user: T,
         relayUrl: NormalizedRelayUrl,