Code review fixes from 8.0: userstat

Backport code review fixes from 8.0:

• (5.6) get_key_user_stats, free_user_stats, init_user_stats, get_key_thread_stats, (+ 5.7) free_thread_stats, init_thread_stats, free_table_stats, get_key_index_stats, free_index_stats should be static to sql_connect.cc
• (5.6) get_key_user_stats, get_key_thread_stats, get_key_table_stats, get_key_index_stats, free_index_stats do not have to be extern "C".
• (5.6, 5.7, 8.0) init_global_thread_stats referencing client_stats in the error message
• (5.6, 5.7) THD::get_client_host_port no longer used