Refactor code of MqttBufferWriter

* Avoid to call MqttPacketBuffer.Join() when SendPacketAsync

* Safely cleanup the PacketFormatterAdapter

* Keep buffer.Join when mqtt over webSocket

* null checking for IHttpContextFeature.HttpContext

Extend reset client statistic to reset all stats

dispose the AsyncSignalWaiter before setting it to null to prevent memory leak
Co-authored-by: NMario Zelger <mario.zelger@confer.ch>

1546 possible bug in server repeatedly sending the same message to client subscribed with cleansession=false (#1569)

Also cleanup `MqttClientSessionsManager._subscriberSessions` when session is taken over by a new one with CleanSession=true, fix #1552 (#1553)

1552 mqttnetservermqttclientsessionsmanager subscribersessions is not proper removed on special case (#1562)

fix cross access to async events.