Improve user statistics accuracy

Currently userstat measures times for mysql_parse, mysqld_stmt_reset, mysqld_stmt_fetch, mysqld_stmt_prepare, mysqld_stmt_execute
MariaDB measures times only for `dispatch_command` which call above functions.
Basically MariaDB adds measurements for COM_INIT_DB, COM_REGISTER_SLAVE, COM_RESET_CONNECTION, and other in `dispatch_command`.
MariaDB's solution is better and more accurate as definition of BUSY_TIME and CPU_TIME.