Columns added to TABLE_STATISTICS
- ROWS_INSERTED, ROWS_DELETED, ROWS_UPDATED, KEY_READ_HITS and
  KEY_READ_MISSES.

Columns added to CLIENT_STATISTICS and USER_STATISTICS:
- KEY_READ_HITS and KEY_READ_MISSES.

User visible changes (except new columns):
- CLIENT_STATISTICS and USER_STATISTICS has columns KEY_READ_HITS and
  KEY_READ_MISSES added after column ROWS_UPDATED before SELECT_COMMANDS.

Other changes:
- Do not collect table statistics for system tables like index_stats
  table_stats, performance_schema, information_schema etc as the user
  has no control of these and the generate noice in the statistics.
- All row variables that are part of user_stats are moved to
  'struct rows_stats' to make it easy to clear all of them at once.
- ha_read_key_misses added to STATUS_VAR

Notes:
- userstat.result has a change of numbers of rows for handler_read_key.
  This is because use-stat-tables is now disabled for the test.