a small bug when use quic_client test connection migration with fixed client_connection_id_length

@bnc-google @wu-bin @vasilvv hi，when I use quic_client test connection migration with fixed client_connection_id_length=16，I find a smalle bug：

In the quic_client, when you specify a fixed client_connection_id_length, will call the function QuicConnection::set_client_connection_id。then the client_connection_id_length will be used to call SetExpectedClientConnectionIdLength to set expected_client_connection_id_length_, which is used to decide dst id length when a packet comes in.

On the other hand, the client issued client id with a default fixed length is 8，when connection migration happens, the server will choose an unused client id with length is 8 to send packet to client。 so when then expected_client_connection_id_length_ is not 8, the client will get a wrong length dst id.