HANG attaching to app that continually creates threads

A 3rd-party app that continually creates threads (similar to api.detach_spawn and other tests of ours, which are indeed flaky) can hang during static-DR attach: it hits the loop added for #2603 (closed) waiting on uninit_thread_count.

One solution is a full #1305 barrier where no threads make progress until all are attached, but I am worried about performance there for apps with many threads. Instead I'm putting in a new-thread barrier where no new threads in taken-over threads are allowed until attach is finished.