#108317: 1. use per-chain mutex instead of global mutex to reduce
lock collision.
- Fix two race conditions. One is between _umtx_unlock and signal, also a thread was marked TDF_UMTXWAKEUP by _umtx_unlock, it is possible a signal delivered to the thread will cause msleep returns EINTR, and the thread breaks out of loop, this causes umtx ownership is not transfered to the thread. Another is in _umtx_unlock itself, when the function sets the umtx to UMTX_UNOWNED state, a new thread can come in and lock the umtx, also the function tries to set contested bit flag, but it will fail. Although the function will wake a blocked thread, if that thread breaks out of loop by signal, no contested bit will be set.