Previously, IMMV including IMMV in its definition can be created by
create_immv(), but it should not be supported by IMMV because we
cannot maintain it recursively for now. This patch prevents it by raising
an error for such view definition on create_immv().
refresh_immv(immv_name, with_data) is a function to refresh IMMV like
REFRESH MATERIALIZED VIEW command. It has two argument.
immv_name is incrementally maintainable materialized view's name, and
with_data is an option that is corresponding to the WITH [NO] DATA option.
When with_data is set false, the IMMV gets unpopulated.
One of differences between IMMVs unpopulated by this function and
normal materialized views unpopulated by REFRESH ... WITH NO DATA
is that such IMMVs can be referenced by SELECT but return no rows,
while unpopulated materialized views are not scanable.
The behaviour may be changed in future to raise an error when unpopulated
an IMMV is scanned.
