model: Change resource.usage_tokens from list to dict

this way we can have efficient look-ups by token UUID

osmocom/lpmgd/model.py

@@ -125,7 +125,7 @@ class Resource:
         self.description = desc
         self.status = "unknown"
         self.use_count = 0
-        self.usage_tokens = []
+        self.usage_tokens = {}
         self.determine_status()
 
     def usage_inc(self):
@@ -144,15 +144,21 @@ class Resource:
     def usage_token_get(self, user_name: str, usage: str, duration_s: int) -> UsageToken:
         """Create/obtain a new usage token."""
         token = UsageToken(self, user_name, usage, duration_s)
-        self.usage_tokens.append(token)
+        self.usage_tokens[token.uuid.hex] = token
         self.usage_inc()
         return token
 
     def usage_token_put(self, token: UsageToken):
         """Release/put an existing usage token."""
-        self.usage_tokens.delete(token)
+        uuid_hex = token.uuid.hex
+        del self.usage_tokens[uuid_hex]
         self.usage_dec()
 
+    def usage_token_find(self, uuid_hex: str):
+        if uuid_hex in self.usage_tokens:
+            return self.usage_tokens[uuid_hex]
+        return None
+
     def get_status(self):
         """Return the (cached) status of the resource."""
         return self.status