Msgr: Add unittests for Messenger

This UT want to ensure we can the same behavior with calling Messenger's method, it contains:

1. basic connection check
2. name and addr check
3. feature support check
4. policy check

At last, because test program need to get whether connection is built. Connection::is_conencted() should be used for this goal. But PipeConnection used "pipe==NULL" which doesn't has any meaning for connection status I think. So here change the PipeConnection impl to suit its actual meaning.

Impl #10147