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/include/FreeRTOS_IP.h

@@ -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
@@ -275,6 +275,15 @@ void FreeRTOS_ClearARP( void );
 #endif /* ipconfigDHCP_REGISTER_HOSTNAME */
 
 
+/* This xApplicationGetRandomNumber() will set *pulNumber to a random number,
+and return pdTRUE. When the random number generator is broken, it shall return
+pdFALSE.
+The function is defined in 'iot_secure_sockets.c'.
+If that module is not included in the project, the application must provide an
+implementation of it.
+The macro's ipconfigRAND32() and configRAND32() are not in use anymore. */
+BaseType_t xApplicationGetRandomNumber( uint32_t *pulNumber );
+
 /* For backward compatibility define old structure names to the newer equivalent
 structure name. */
 #ifndef ipconfigENABLE_BACKWARD_COMPATIBILITY