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/mu-query-match-deciders.hh

@@ -0,0 +1,85 @@
+/*
+** 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.
+**
+*/
+
+
+#ifndef MU_QUERY_MATCH_DECIDERS_HH__
+#define MU_QUERY_MATCH_DECIDERS_HH__
+
+#include <unordered_set>
+#include <unordered_map>
+#include <memory>
+
+#include <xapian.h>
+
+#include "mu-query-results.hh"
+
+
+namespace Mu {
+using StringSet = std::unordered_set<std::string>;
+
+
+struct DeciderInfo {
+        QueryMatches matches;
+        StringSet    thread_ids;
+        StringSet    message_ids;
+};
+
+/**
+ * Make a "leader" decider, that is, a MatchDecider for either a singular or the
+ * first query in the leader/related pair of queries. Gather information for
+ * threading, and the subsequent "related" query.
+*
+ * @param qflags     query flags
+ * @param match_info receives information about the matches.
+ *
+ * @return a unique_ptr to a match decider.
+ */
+std::unique_ptr<Xapian::MatchDecider>  make_leader_decider(QueryFlags qflags,
+                                                           DeciderInfo& info);
+
+
+/**
+ * Make a "related" decider, that is, a MatchDecider for the second query
+ * in the leader/related pair of queries.
+ *
+ * @param qflags     query flags
+ * @param match_info receives information about the matches.
+ *
+ * @return a unique_ptr to a match decider.
+ */
+std::unique_ptr<Xapian::MatchDecider>  make_related_decider(QueryFlags qflags,
+                                                            DeciderInfo& info);
+
+
+/**
+ * Make a "final" decider, that is, a MatchDecider that removes all but
+ * the document excepts for the ones included earlier.
+ *
+ * @param qflags     query flags
+ * @param match_info receives information about the matches.
+ *
+ * @return a unique_ptr to a match decider.
+ */
+std::unique_ptr<Xapian::MatchDecider> make_final_decider (QueryFlags qflags,
+                                                          DeciderInfo& info);
+
+
+} // namepace Mu
+
+#endif /* MU_QUERY_MATCH_DECIDERS_HH__ */