sql: fix resulting type calculation for CASE-WHEN stmt

Before this patch, resulting type for CASE-WHEN statement was assumed to
be the same as type of argument of first THEN clause. Obviously, it is
wrong and could lead to sad consequence (e.g. creating ephemeral table
with inconsistent format). To deal with this, we check all THEN
arguments: if all of them have the same type, then such type will be
resulting of the whole statement; if at least two types are different,
we can't determine actual resulting type during compilation stage and
assign SCALAR as a most general type in SQL now.

Need for #4206