added dedicated test file and moved timed_cache to tools

libp2p/tools/timed_cache/first_seen_cache.py

@@ -0,0 +1,20 @@
+import time
+
+from .base_timed_cache import (
+    BaseTimedCache,
+)
+
+
+class FirstSeenCache(BaseTimedCache):
+    """Cache where expiry is set only when first added."""
+
+    def add(self, key: bytes) -> bool:
+        with self.lock:
+            if key in self.cache:
+                return False
+            self.cache[key] = int(time.time()) + self.ttl
+            return True
+
+    def has(self, key: bytes) -> bool:
+        with self.lock:
+            return key in self.cache