Add Connection::reset

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

src/connection.cpp

@@ -118,10 +118,6 @@ bool Connection::writeBytes() {
   }
   assert(messages_.empty());
 
-  // Reset arena after completing request/response cycle
-  // This reclaims memory from request parsing and response generation
-  arena_.reset();
-
   return false;
 }