Add chainstate obfuscation to avoid spurious antivirus detection

Adds an `obfuscate` parameter to `CLevelDBWrapper` and makes use of it
for all new chainstate stores built via `CCoinsViewDB`. Also adds an
`Xor` method to `CDataStream`.

Thanks to @sipa @laanwj @pstratem @dexX7 @KyrosKrane @gmaxwell.

src/test/streams_tests.cpp

@@ -0,0 +1,67 @@
+// Copyright (c) 2012-2013 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "streams.h"
+#include "support/allocators/zeroafterfree.h"
+#include "test/test_bitcoin.h"
+
+#include <boost/assign/std/vector.hpp> // for 'operator+=()'
+#include <boost/assert.hpp>
+#include <boost/test/unit_test.hpp>
+                    
+using namespace std;
+using namespace boost::assign; // bring 'operator+=()' into scope
+
+BOOST_FIXTURE_TEST_SUITE(streams_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(streams_serializedata_xor)
+{
+    std::vector<char> in;
+    std::vector<char> expected_xor;
+    std::vector<unsigned char> key;
+    CDataStream ds(in, 0, 0);
+
+    // Degenerate case
+    
+    key += '\x00','\x00';
+    ds.Xor(key);
+    BOOST_CHECK_EQUAL(
+            std::string(expected_xor.begin(), expected_xor.end()), 
+            std::string(ds.begin(), ds.end()));
+
+    in += '\x0f','\xf0';
+    expected_xor += '\xf0','\x0f';
+    
+    // Single character key
+
+    ds.clear();
+    ds.insert(ds.begin(), in.begin(), in.end());
+    key.clear();
+
+    key += '\xff';
+    ds.Xor(key);
+    BOOST_CHECK_EQUAL(
+            std::string(expected_xor.begin(), expected_xor.end()), 
+            std::string(ds.begin(), ds.end())); 
+    
+    // Multi character key
+
+    in.clear();
+    expected_xor.clear();
+    in += '\xf0','\x0f';
+    expected_xor += '\x0f','\x00';
+                        
+    ds.clear();
+    ds.insert(ds.begin(), in.begin(), in.end());
+
+    key.clear();
+    key += '\xff','\x0f';
+
+    ds.Xor(key);
+    BOOST_CHECK_EQUAL(
+            std::string(expected_xor.begin(), expected_xor.end()), 
+            std::string(ds.begin(), ds.end()));  
+}         
+
+BOOST_AUTO_TEST_SUITE_END()