1. rename semphone to semaphore
2. add comment for tsched.h
3. change the function signature for taosSchedulerTask, changing from
return int to void. We currently don't check any return code of the function.
4. add some error handlings. For fatal error, just exit the process because
the program may run into a random state.

2. semaphore rather than dispatch_semaphore for tsem_xxx
3. fail-fast comments
4. niche