alter: allow to restrict space format of a non-empty space

At first, now if a space is not empty, its format can not be
restricted by nullability removal, or by restriction of a field type.

The workaround was format removal and reset. Lets allow normal format
restriction followed by space format checking.

At second, now an index is rebuild on any change of its key definition.
But there is no sense to rebuild it, if uniqueness is turned off, part
field numbers and collations are not changed, because it means, that
only types was restricted or extended. So format checking is enough.

Needed for #3008