query: Rework querying/threading machinery

Rewrite the query machinery in c++:
- use an MSet decorator instead of the mu-msg-iter stuff
- use mu-query-decider to mark duplicates/unreadable/related messages
- use mu-query-threader to replace the older container/thread code

Algorithm did not substantially change, but the implementation details
did.

lib/test-query.cc

@@ -0,0 +1,91 @@
+/*
+** Copyright (C) 2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+#include <config.h>
+
+#include <vector>
+#include <glib.h>
+
+#include <iostream>
+#include <sstream>
+#include <unistd.h>
+
+#include "mu-store.hh"
+#include "mu-query.hh"
+#include "index/mu-indexer.hh"
+#include "utils/mu-utils.hh"
+#include "test-mu-common.hh"
+
+using namespace Mu;
+
+static void
+test_query()
+{
+        allow_warnings();
+
+	Store store{test_mu_common_get_random_tmpdir(), std::string{MU_TESTMAILDIR}, {},{}};
+        auto&& idx{store.indexer()};
+
+	g_assert_true (idx.start(Indexer::Config{}));
+        while (idx.is_running()) {
+                sleep(1);
+        }
+
+        auto dump_matches=[](const QueryResults& res) {
+                size_t n{};
+                for (auto&& item: res)
+                        g_debug ("%02zu %s %s", ++n, item.path().value_or("<none>").c_str(),
+                                 item.message_id().value_or("<none>").c_str());
+        };
+
+
+        Query q{store};
+        g_assert_cmpuint(store.size(),==,19);
+
+        {
+                const auto res = q.run("", MU_MSG_FIELD_ID_NONE, QueryFlags::None);
+                g_assert_true(!!res);
+                g_assert_cmpuint(res->size(),==,19);
+                dump_matches(*res);
+        }
+
+        {
+                const auto res = q.run("", MU_MSG_FIELD_ID_PATH, QueryFlags::None, 11);
+                g_assert_true(!!res);
+                g_assert_cmpuint(res->size(),==,11);
+                dump_matches(*res);
+        }
+}
+
+int
+main (int argc, char *argv[]) try
+{
+	g_test_init (&argc, &argv, NULL);
+
+	g_test_add_func ("/query", test_query);
+
+	return g_test_run ();
+
+
+} catch (const std::runtime_error& re) {
+	std::cerr << re.what() << "\n";
+	return 1;
+} catch (...) {
+        std::cerr << "caught exception\n";
+        return 1;
+}