Drop user defined functions in `rake gitlab:db:drop_tables`

What does this MR do and why?

Previously the backup restore Rake task would fail if the schema were loaded and then dropped via rake gitlab:db:drop_tables because the task would not clean up the 13 user-defined functions in the schema. The CREATE FUNCTION in db/structure.sql would fail the restore because it would collide with an existing function.

Since DROP DATABASE may need superuser permissions, the backup restore task expects that rake gitlab:db:drop_tables brings the database into a clean state. To ensure that's the case, we now search for all user defined functions and drop them. Note that while most of the functions are used with triggers, next_traversal_ids_sibling is a noteable exception.

Relates to #356032 (closed)

How to set up and validate locally

1. Run RAILS_ENV=test bundle exec rake db:reset
2. gdk psql -d gitlabhq_test
3. See the list of functions: \df+
4. See the list of plpgsql functions:

SELECT p.proname FROM pg_catalog.pg_proc p
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
     LEFT JOIN pg_catalog.pg_language l ON l.oid = p.prolang
WHERE pg_catalog.pg_function_is_visible(p.oid)
      AND n.nspname <> 'pg_catalog'
      AND n.nspname <> 'information_schema'
      AND l.lanname = 'plpgsql';

5. Run RAILS_ENV=test bundle exec rake gitlab:db:drop_tables.
6. Repeat steps 3 and 4. You should all the plpgsql functions wiped.
7. Repeat step 1 to bring your database back to normal.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.