Fix Swoole in-memory timer table size

[miguilimzero]

The current implementation of the timer table starts to throw some insufficient memory errors when there are a lot of workers ~200 and when all those workers are processing requests at the same time (as reported at #648).

The #650 PR introduced the max_timer_table_size undocumented config value so we can control the timer table size manually to avoid this problem.

However, ensuring the $size passed to the table creation will guarantee the desired number of rows (with #818). We can now use the number of workers to define the timer table size and get rid of the undocumented manual parameter.

[taylorotwell]

I don't understand why we wouldn't just document the configuration option instead?