Datetime `set()` timestamp type check fixed.

[TarantoolBot]

For backward compatibility
the compat.datetime_setfn_timestamp_type_check option has been
introduced. It's disabled by default for now ('old' behaviour),
which means no type check is performed. The 'new' behaviour
(with type check) is planned to be set as the default in version 4.x.

Required docu page: https://tarantool.io/compat/datetime_setfn_timestamp_type_check

See DATETIME_SETFN_TIMESTAMP_TYPE_CHECK_BRIEF in compat.lua for details.
Requested by @tmr-g in tarantool/tarantool@df461f1.