Disallow using space_id

[DifferentialOrange]

crud.len allows to using space_id instead of space_name:

crud/crud/len.lua

Lines 39 to 40 in 76e3374

	function len.call(space_name, opts)
	checks('string|number', {

AFAIK, every other crud method do not support it, so there is an inconsistency. There are two possible ways, and it seems like it is better to prohibit using space_id since in sharded system its consistency is not guaranteed: see @olegrok #244 (comment)