read_statistics_for_tables_if_needed

Regression after 279a907f, read_statistics_for_tables_if_needed() was
called after open_normal_and_derived_tables() failure.

Fixed by moving read_statistics_for_tables() call to a branch of
get_schema_stat_record() where result of open_normal_and_derived_tables()
is checked.

Removed THD::force_read_stats, added read_statistics_for_tables() instead.
Simplified away statistics_for_command_is_needed().