Change Item_bool_rowready_func2 to cache const items at fix time rather than evaluation time

Currently there is an asymmetry in how binary comparison operators (such as = or >, etc) are implemented for temporal vs other data types.

Item_bool_rowready_func2 does not cache its constant arguments of temporal types at fix time. Instead, constant arguments are replaced to instances of Item_cache_temporal at evaluation time, when get_datetime_value() is executed for the first time.

Non-temporal types do it in a different way: constant arguments are cached at fix time, while the evaluation time code does not do any argument substitution or caching. Substitution is done in Arg_comparator::set_cmp_func_(string|int|real|decimals)(), with these two lines:

  a= cache_converted_constant(thd, a, &a_cache, compare_type_handler());

  b= cache_converted_constant(thd, b, &b_cache, compare_type_handler());

Under terms of this task we will change the code for the temporal data types to cache constant arguments in fix time, using cache_converted_constant(), like it is done for the other data types. Caching during evaluation time will be removed.

Rationale:

• We're adding pluggable data types soon. It's better to have symmetric code, to add new data types easier.
• TIMESTAMP should be fixed to do comparison in my_time_t format internally, without conversion to MYSQL_TIME (such conversion can be lossy, see MDEV-13995). It would be nice to make the code symmetric before fixing bugs like MDEV-13995, for simplicity.