lib: improve threaded sorting

Improve threaded sorting:
- rework the recursive algo (which should a crash noted earlier)
- sort by thread key (as in 1.5.7)

Fixes: #1923
Fixes: #1922
Fixes: #1911
Fixes: #1916

lib/mu-server.cc

@@ -139,6 +139,7 @@ add_thread_info (Sexp::List& items, const QueryMatch& qmatch)
 
         info.add_prop(":path",  Sexp::make_string(qmatch.thread_path));
         info.add_prop(":level", Sexp::make_number(qmatch.thread_level));
+        info.add_prop(":date",  Sexp::make_string(qmatch.thread_date));
 
         if (qmatch.has_flag(QueryMatch::Flags::Root))
                 info.add_prop( ":root", symbol_t());