v2.8

• 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