Update helper setup to use DROP FUNCTION IF EXISTS

[seanlinsley]

If the return type of a function changes, CREATE OR REPLACE FUNCTION will fail with ERROR: cannot change return type of existing function. To make function setup more foolproof, this PR updates the documentation to instead use DROP FUNCTION IF EXISTS.

Note that I've only updated get_column_stats and get_relation_stats_ext since they return a subset of columns, while get_stat_replication for example does SELECT *, so shouldn't be affected when Postgres adds new columns in the future.