refactor: do some internal refactor, and set the correct intermediate buffer size for average function.

enh(query): filter the child table that are not belongs to current super table when adding into candidate list.

fix(query): fix the bug that special pseudo column tbname, along with other normal columns in select clause, cause taosd crash.

fix(query): revise the output data column info in the exchange operator to be adaptable for the case of extensive datablocks.