AsyncMessenger: Fix wrong in_seq from server

In AsyncMessenger, server may maintain legacy client state(in_seq), if a fresh
client connect the server with existing connection, server may reply with a
seq which is unknown for client. Client just need to ignore it.

This behavior is a little different from SimpleMessenger, server-side
connection with SimpleMessenger will discard all states if client "stopped".

AsyncMessenger has a lightweight connection model(instead of thread for
SimpleMessenger) so it want to avoid frequency Connection alloc/dealloc by
make connection standby in memory instead of reaping in SimpleMessenger.

Fix #10022

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>