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/Makefile.am

@@ -170,10 +170,10 @@ TEST_PROGS+=test-mu-tokenizer
 test_mu_tokenizer_SOURCES=test-tokenizer.cc
 test_mu_tokenizer_LDADD=libtestmucommon.la
 
-# TEST_PROGS+=test-mu-threader
-# test_mu_threader_SOURCES=mu-query-threader.cc
-# test_mu_threader_LDADD=libtestmucommon.la
-# test_mu_threader_CXXFLAGS=$(AM_CXXFLAGS) -DBUILD_THREADER_TEST
+TEST_PROGS+=test-mu-threader
+test_mu_threader_SOURCES=mu-query-threader.cc
+test_mu_threader_LDADD=libtestmucommon.la
+test_mu_threader_CXXFLAGS=$(AM_CXXFLAGS) -DBUILD_THREADER_TEST
 
 TEST_PROGS+=test-mu-parser
 test_mu_parser_SOURCES=test-parser.cc