* MqttRpcClient now adds ResponseTopic to MqttApplicationMessage when using MQTT v5

* No need for conditional protocol check (ResponseTopic property won't be added by PacketFormatter in <MQTT v5)

* Cleanup: Remove using

fix order of parameters in exceptions AreEqual(expected, actual)

* Fix timestamp check in server keep-alive monitor

* Retain the original client statistics property name
- add comments to clarify meaning
- add separate public "received" timestamp property

* fix comment

* Set IsSessionPresent in CONNACK if client session exists

* Set IsSessionPresent in CONNACK if client session exists;
- restore white space differences to make comparison easier

* Add support for RetainHandling.

* Add support for No Local.

* Add support for _SubscriptionIdentifier_.

* Add more Unit Tests.

* Update docs.

* Updated tests with public servers.

* Separated tests with TLS12 and TLS13.

for all four combinations of null/non-null for username and password

.Net5.0 TFMs added and related nuget packages updated.