Add Connection::reset

There was an issue where the connection deque was referring to old arena memory

design.md

@@ -121,7 +121,6 @@ Key features:
 
 Key features:
 - Arena allocator per connection for efficient memory management
-- **Request/Response arena lifecycle**: Arena resets after each complete request/response cycle
 - Weak reference to server for safe cleanup after server destruction
 - Private networking details accessible only to Server via friend relationship
 - Public handler interface: appendMessage(), closeAfterSend(), getArena(), getId()
@@ -180,9 +179,10 @@ Arena Memory Lifecycle:
 2. **Response Generation**: Handler uses arena for temporary response construction (headers, JSON, etc.)
 3. **Response Queuing**: Handler calls `conn->appendMessage()` which copies data to arena-backed message queue
 4. **Response Writing**: Server writes all queued messages to socket via `writeBytes()`
-5. **Arena Reset**: After successful write completion, arena resets to reclaim all memory from the request/response cycle
 
-This design assumes request/response pairs (HTTP-like protocols) but works for any protocol where there's a clear completion point for memory reclamation.
+The user is responsible for calling `conn->reset()` periodically to reclaim the
+arena memory. A good time to do this for a request/response protocol would be
+after all outgoing bytes have been written.
 
 ### API Design