refactor: replace Boost shared_mutex with std shared_mutex in sigcache

Co-authored-by: MarcoFalke falke.marco@gmail.com
Co-authored-by: sinetek pitwuu@gmail.com

test/lint/lint-includes.sh

@@ -67,8 +67,6 @@ EXPECTED_BOOST_INCLUDES=(
     boost/signals2/optional_last_value.hpp
     boost/signals2/signal.hpp
     boost/test/unit_test.hpp
-    boost/thread/lock_types.hpp
-    boost/thread/shared_mutex.hpp
 )
 
 for BOOST_INCLUDE in $(git grep '^#include <boost/' -- "*.cpp" "*.h" | cut -f2 -d: | cut -f2 -d'<' | cut -f1 -d'>' | sort -u); do