Protect websockets_ by mutex

[andrew-guz]

We faced the situation, when std::vector<std::shared_ptr<websocket::connection>> websockets_ will be modified in several threads in case of several simultaneous connections been opened and closed.

To prove this and for future needs, we added ThreadSanitizer support (ENABLE_TSAN flag) and new test (multithreaded_websockets_open_close) that shows this problem:

WARNING: ThreadSanitizer: data race (pid=305193)
  Read of size 8 at 0x7ffd1dd3bb48 by main thread:
    #0 __gnu_cxx::__normal_iterator<std::shared_ptr<crow::websocket::connection>*, std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > > >::__normal_iterator(std::shared_ptr<crow::websocket::connection>* const&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/bits/stl_iterator.h:1059 (unittest+0x324e8b) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)
    #1 std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > >::end() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/bits/stl_vector.h:1019 (unittest+0x324e8b)
    #2 crow::Crow<>::close_websockets() /Crow/include/crow/app.h:623 (unittest+0x324a57) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)
    #3 crow::Crow<>::stop() /Crow/include/crow/app.h:614 (unittest+0x26352e) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)

  Previous write of size 8 at 0x7ffd1dd3bb48 by thread T2:
    #0 std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > >::_M_erase_at_end(std::shared_ptr<crow::websocket::connection>*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/bits/stl_vector.h:2240 (unittest+0x62b6fc) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)
    #1 std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > >::_M_erase(__gnu_cxx::__normal_iterator<std::shared_ptr<crow::websocket::connection>*, std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > > >, __gnu_cxx::__normal_iterator<std::shared_ptr<crow::websocket::connection>*, std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > > >) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/bits/vector.tcc:202 (unittest+0x62b29d) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)
    #2 std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > >::erase(__gnu_cxx::__normal_iterator<std::shared_ptr<crow::websocket::connection> const*, std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > > >, __gnu_cxx::__normal_iterator<std::shared_ptr<crow::websocket::connection> const*, std::vector<std::shared_ptr<crow::websocket::connection>, std::allocator<std::shared_ptr<crow::websocket::connection> > > >) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/bits/stl_vector.h:1824 (unittest+0x62ad7f) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)
    #3 crow::Crow<>::remove_websocket(std::shared_ptr<crow::websocket::connection>) /Crow/include/crow/app.h:640 (unittest+0x62a908) (BuildId: e1fdcc2ba235601f5098b0fb0206a07bdd014f10)

So, protected websockets_ usage with new mutex.