BIP152: Allow block responses to getblocktxn requests

Corresponding bitcoind change in:

dac53b5 Modify getblocktxn handler not to drop requests for old blocks

bip-0152.mediawiki

@@ -128,7 +128,7 @@ A new inv type (MSG_CMPCT_BLOCK == 4) and several new protocol messages are adde
 
 ====getblocktxn====
 # The getblocktxn message is defined as as a message containing a serialized BlockTransactionsRequest message and pchCommand == "getblocktxn".
-# Upon receipt of a properly-formatted getblocktxn message, nodes which recently provided the sender of such a message a cmpctblock for the block hash identified in this message MUST respond with an appropriate blocktxn message. Such a blocktxn message MUST contain exactly and only each transaction which is present in the appropriate block at the index specified in the getblocktxn indexes list, in the order requested.
+# Upon receipt of a properly-formatted getblocktxn message, nodes which recently provided the sender of such a message a cmpctblock for the block hash identified in this message MUST respond with either an appropriate blocktxn message, or a full block message. A blocktxn response MUST contain exactly and only each transaction which is present in the appropriate block at the index specified in the getblocktxn indexes list, in the order requested.
 
 ====blocktxn====
 # The blocktxn message is defined as as a message containing a serialized BlockTransactions message and pchCommand == "blocktxn".