Convert conversation id to a UUID

This UUID is only needed to be unique for any pair of
(Alice,Bob) having a conversation. In the future when
group chats are supported, we will need them to be unique
for all users in the chat. We do not rely on this ID
to be globally unique for all Hush chats in any way.

Without a conversation id, Charlie the attacker can
attempt to impersonate Alice or Bob if he knows some or
all of the zaddrs involved. If the conversation id was
predictable, such as the SHA256 hash of both zaddrs,
Charlie could predict it if he knew both zaddrs,
which is likely to happen in many attack scenarios.

If cid had a small number of possible values, the attacker
could "grind" all of them and eat the cost of the extra
tx fees, so cid must have a large state space. QUuid gives
us essentially a random 128bit number, which is twice
the protection as a random int64.

This mitigation will not be needed once HIP304 and
z_signmessage/z_verifymessage are complete, which allow
us to have fully authenticated and signed messages from zaddrs.

src/mainwindow.cpp

@@ -193,8 +193,8 @@ void MainWindow::sendMemo() {
     //TODO: verify we currently own the private key to this zaddr via z_validateaddress
     tx.fromAddr = chat.getMyZaddr();
     double amount = 0;
-    //TODO: cid=random int64 or sha256
-    QString cid = QString::number( time(NULL) % std::rand() ); // low entropy for testing!
+    //QString cid = QString::number( time(NULL) % std::rand() ); // low entropy for testing!
+    QString cid = QUuid::createUuid().toString(QUuid::WithoutBraces);
     QString hmemo= createHeaderMemo(cid,chat.getMyZaddr());
     QString memo = ui->textEdit->toPlainText();
     QString addr = contact.getZaddr();