Update references to max_num_X

In the context of cbepctl set flush_param

the following parameters

max_num_writers

max_num_readers

max_num_auxio

max_num_nonio

are being deprecated in favour of

num_writer_threads

num_reader_threads

num_nonio_threads

num_auxio_threads

and will be removed in a future version.

This is to reflect that these commands now actually dynamically alter the number of threads for each task type.

Similarly, the statistics

ep_max_num_writers

ep_max_num_readers

ep_max_num_auxio

ep_max_num_nonio

are being deprecated in favour of

ep_num_writer_threads

ep_num_reader_threads

ep_num_nonio_threads

ep_num_auxio_threads

and will be removed in a future version.
The only place I have found these in the documentation is cbstats all.