Add skeleton chain and client classes

This commit does not change behavior. It just adds new skeleton classes that
don't do anything and aren't instantiated yet.

src/interfaces/chain.cpp

@@ -0,0 +1,20 @@
+// Copyright (c) 2018 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 <interfaces/chain.h>
+
+#include <util/system.h>
+
+namespace interfaces {
+namespace {
+
+class ChainImpl : public Chain
+{
+};
+
+} // namespace
+
+std::unique_ptr<Chain> MakeChain() { return MakeUnique<ChainImpl>(); }
+
+} // namespace interfaces