crimson/mgr: close() in background

crimson/mgr: close() in background

as per Yingxin,

application code is not required to wait for the `close()` future, it
would be safe to ignore it, because:
- `close()` will shutdown its socket synchronously;
- `close()` will create an internal `ConnectionRef` when it's closing;
- `Messenger` will wait for all connections closed during `shutdown()`;

Signed-off-by: Kefu Chai <kchai@redhat.com>