AsyncConnection: Avoid hungry if mark_down's caller is the eventcenter's owner

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
AsyncConnection: Avoid name conflicting with AsyncMessenger

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
AsyncMessenger: Fix accept connection replacing process

Original there exists bugs that we don't modify existing file event,
it will let remaining alive connection's event won't be called by
EventCenter.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
AsyncConnection: Add loopback connection support

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>