The lts_builder add_transition should not be called from different threads, even with a mutex

[mlaveaux]

Describe the bug
Calling add_transition from a different thread can result in multi_action terms being moved from a thread that does not own this term.

Solution
I think the solution is that utilities::unordered_map_large<lps::multi_action, std::size_t> m_actions; should be an atermpp::unordered_map_large.