refactor: Move src/interfaces/*.cpp files to libbitcoin_common.a

These belong in libbitcoin_common.a, not libbitcoin_util.a, because they aren't
general-purpose utilities, they just contain common code that is used by both
the node and the wallet. Another reason to reason to not include these in
libbitcoin_util.a is to prevent them from being used by the kernel library.

src/common/interfaces.cpp

@@ -0,0 +1,53 @@
+// Copyright (c) 2021 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/echo.h>
+#include <interfaces/handler.h>
+
+#include <boost/signals2/connection.hpp>
+#include <memory>
+#include <utility>
+
+namespace common {
+namespace {
+class CleanupHandler : public interfaces::Handler
+{
+public:
+    explicit CleanupHandler(std::function<void()> cleanup) : m_cleanup(std::move(cleanup)) {}
+    ~CleanupHandler() override { if (!m_cleanup) return; m_cleanup(); m_cleanup = nullptr; }
+    void disconnect() override { if (!m_cleanup) return; m_cleanup(); m_cleanup = nullptr; }
+    std::function<void()> m_cleanup;
+};
+
+class HandlerImpl : public interfaces::Handler
+{
+public:
+    explicit HandlerImpl(boost::signals2::connection connection) : m_connection(std::move(connection)) {}
+
+    void disconnect() override { m_connection.disconnect(); }
+
+    boost::signals2::scoped_connection m_connection;
+};
+
+class EchoImpl : public interfaces::Echo
+{
+public:
+    std::string echo(const std::string& echo) override { return echo; }
+};
+} // namespace
+} // namespace common
+
+namespace interfaces {
+std::unique_ptr<Handler> MakeHandler(std::function<void()> cleanup)
+{
+    return std::make_unique<common::CleanupHandler>(std::move(cleanup));
+}
+
+std::unique_ptr<Handler> MakeHandler(boost::signals2::connection connection)
+{
+    return std::make_unique<common::HandlerImpl>(std::move(connection));
+}
+
+std::unique_ptr<Echo> MakeEcho() { return std::make_unique<common::EchoImpl>(); }
+} // namespace interfaces