[td-5418]<enhance>: change the refresh thread generate mechanism to reduce the number of refresh thread.

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.

[TD-4243]<enhance>: block_dist percentile approximation to reduce memory usage especially for super table