* Elide repeated Action-delegate allocation by caching it

Here C# compiler has too little knowledge about the usage of the delegate, we know better so cache it on our own to avoid the repeated allocation.

* Use conditional compilation

* 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 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.

* process all queued message befor server stops

* fix an issue

Co-authored-by: ablfzl <ablfzl21@outlook.com>

* Added option for how many subscriptions to send in a single call to the server.  aws iotcore limits this to 8

* Split subscription messages up to 8 at a time when reconnecting

update function WithMessageExpiryInterval description

Based on https://github.com/chkr1011/MQTTnet/issues/1138#issuecomment-846649937

Fix #996
Fix #1010