Sync FreeRTOS-Plus/Source/FreeRTOS-Plus-TCP with the version in GitHub at (23665258cabe49d5d68ba23968b6845a7c80eb34).

Notes: 
- header has version 2.2.0. 
- This sync did not bring in ./test directory, though we should. 
- New NetworkInterfaces are introduced by this merge.
- Keil compiler support. 
- FreeRTOS_IP.h new API xApplicationGetRandomNumber().
- FreeRTOS_IP_Private.h new eIPEvent_t eNetworkTxEvent. 
- FreeRTOS_Stream_Buffer.h removing static xStreamBufferIsEmpty() and xStreamBufferIsFull().
- FreeRTOSConfigDefaults.h provides default ipconfigDNS_RECEIVE_BLOCK_TIME_TICKS. 
- other type changes.

FreeRTOS-Plus/Source/FreeRTOS-Plus-TCP/FreeRTOS_ARP.c

@@ -1,5 +1,5 @@
 /*
- * FreeRTOS+TCP V2.0.11
+ * FreeRTOS+TCP V2.2.0
  * Copyright (C) 2017 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy of
@@ -580,8 +580,24 @@ NetworkBufferDescriptor_t *pxNetworkBuffer;
 			}
 		}
 		#endif
+		if( xIsCallingFromIPTask() != 0 )
+		{
+			/* Only the IP-task is allowed to call this function directly. */
+			xNetworkInterfaceOutput( pxNetworkBuffer, pdTRUE );
+		}
+		else
+		{
+		IPStackEvent_t xSendEvent;
 
-		xNetworkInterfaceOutput( pxNetworkBuffer, pdTRUE );
+			/* Send a message to the IP-task to send this ARP packet. */
+			xSendEvent.eEventType = eNetworkTxEvent;
+			xSendEvent.pvData = ( void * ) pxNetworkBuffer;
+			if( xSendEventStructToIPTask( &xSendEvent, ( TickType_t ) portMAX_DELAY ) == pdFAIL )
+			{
+				/* Failed to send the message, so release the network buffer. */
+				vReleaseNetworkBufferAndDescriptor( pxNetworkBuffer );
+			}
+		}
 	}
 }
 
@@ -589,6 +605,12 @@ void vARPGenerateRequestPacket( NetworkBufferDescriptor_t * const pxNetworkBuffe
 {
 ARPPacket_t *pxARPPacket;
 
+	/* Buffer allocation ensures that buffers always have space
+	for an ARP packet. See buffer allocation implementations 1
+	and 2 under portable/BufferManagement. */
+	configASSERT( pxNetworkBuffer );
+	configASSERT( pxNetworkBuffer->xDataLength >= sizeof(ARPPacket_t) );
+
 	pxARPPacket = ( ARPPacket_t * ) pxNetworkBuffer->pucEthernetBuffer;
 
 	/* memcpy the const part of the header information into the correct