threadsafe_queue: Fix deadlock

This fixes a lost wakeup in SPSCQueue. If the reader is in just the right position, the writer's notification will be lost and this will be a problem if the writer then does something to wait on the reader.

This was discovered to affect my upcoming stacktrace PR. I don't think any performance decrease will be noticeable because an uncontended mutex is smart enough to skip the syscall. This PR might also resolve some rare deadlocks but I don't know of any examples.