Update PubSubClient 2.8 (#1438)

Co-authored-by: Olivier <Olivier@nowhere.org>

Author: tekka007

drivers/PubSubClient/CHANGES.txt

@@ -1,3 +1,12 @@
+2.8
+   * Add setBufferSize() to override MQTT_MAX_PACKET_SIZE
+   * Add setKeepAlive() to override MQTT_KEEPALIVE
+   * Add setSocketTimeout() to overide MQTT_SOCKET_TIMEOUT
+   * Added check to prevent subscribe/unsubscribe to empty topics
+   * Declare wifi mode prior to connect in ESP example
+   * Use `strnlen` to avoid overruns
+   * Support pre-connected Client objects
+
 2.7
    * Fix remaining-length handling to prevent buffer overrun
    * Add large-payload API - beginPublish/write/publish/endPublish

drivers/PubSubClient/LICENSE.txt

@@ -1,4 +1,4 @@
-Copyright (c) 2008-2015 Nicholas O'Leary
+Copyright (c) 2008-2020 Nicholas O'Leary
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the