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/interfaces/handler.cpp

@@ -1,45 +0,0 @@
-// Copyright (c) 2018-2021 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/handler.h>
-
-
-#include <boost/signals2/connection.hpp>
-#include <utility>
-
-namespace interfaces {
-namespace {
-
-class HandlerImpl : public 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 CleanupHandler : public 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;
-};
-
-} // namespace
-
-std::unique_ptr<Handler> MakeHandler(boost::signals2::connection connection)
-{
-    return std::make_unique<HandlerImpl>(std::move(connection));
-}
-
-std::unique_ptr<Handler> MakeHandler(std::function<void()> cleanup)
-{
-    return std::make_unique<CleanupHandler>(std::move(cleanup));
-}
-
-} // namespace interfaces