Add stopClient to close connection without sending disconnect message

[albertov]

This is necessary to ensure the last-will message is delivered by the broker when using protocol MQTT 3.1.1 (which has no disconnect-with-will disconnect reason).

[albertov]

Relevant section of the spec:

On receipt of DISCONNECT the Server:
MUST discard any Will Message associated with the current connection without publishing it, as described in Section 3.1.2.5 [MQTT-3.14.4-3].

http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718090

So sending a DISCONNECT message unconditionally causes a compliant MQTT 3.1.1 broker to discard the will message. This is a similar situation as other MQTT library authors face, eg empicano/aiomqtt#28