Fix deadlock in tests (Continued)

[wezrule]

Following on from #2043 I am still getting a deadlock on Windows from a similar issue where the last node shared_ptr reference is freed while holding the mutex of bootstrap_initiator and the ~bootstrap_initiator destructor is implicitly called (and hence stop) which then has the same problem as before as it then locks the same mutex while already holding it. Made stopped a std::atomic<bool> so that it can be checked without locking the mutex. The thread can already be joined, so have moved this inside the stopped function (when stopped is false) to avoid an already joined error.