Hush Full Node is now GPLv3

Any projects which want to use Hush code from now on will need to be licensed as
GPLv3 or we will send the lawyers: https://www.softwarefreedom.org/

Notably, Komodo (KMD) is licensed as GPLv2 and is no longer compatible to receive
code changes, without causing legal issues. MIT projects, such as Zcash, also cannot pull
in changes from the Hush Full Node without permission from The Hush Developers,
which may in some circumstances grant an MIT license on a case-by-case basis.

qa/rpc-tests/p2p_txexpiry_dos.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python2
 # Copyright (c) 2018 The Zcash developers
-# Distributed under the MIT software license, see the accompanying
-# file COPYING or https://www.opensource.org/licenses/mit-license.php
+# Distributed under the GPLv3 software license, see the accompanying
+# file COPYING or https://www.gnu.org/licenses/gpl-3.0.en.html
 
 from test_framework.mininode import NodeConn, NodeConnCB, NetworkThread, \
     CTransaction, msg_tx, mininode_lock, OVERWINTER_PROTO_VERSION