switched from crossbeam to flume, for async support + fixed Instance::report_err() and Instance::new_connection_join_namespace()

src/core/instance/subscription.rs

@@ -1,6 +1,6 @@
 use core::fmt;
 use std::rc::Rc;
-use crossbeam::channel::Sender;
+use flume::Sender;
 use std::sync::Arc;
 use tracing::trace;