bugfix: schemas/vegbien.sql: locationevent: added missing fkey on place_visit_id

bugfix: schemas/vegbien.sql: _plots_06_list_of_plots_with_stem_measurements: only include stemobservation records which have actual stem IDs, not merely stem-related measurements (DBH, etc.)

bugfix: schemas/vegbien.sql: _plots_06_list_of_plots_with_stem_measurements: LEFT JOIN to project instead of inner joining, to get Postgres to use the right query plan. this is the last change needed to make query #6 runnable.

bugfix: schemas/vegbien.sql: rematerialize_out_view(): run all queries with `SET enable_seqscan = off` to avoid slow query plans. this fixes _plots_06_list_of_plots_with_stem_measurements and significantly speeds up _plots_10_count_of_individuals_per_plot_in_each_project (and possibly others).

schemas/vegbien.sql: locationevent: documented `CREATE INDEX locationevent_place_visit_id` runtime (3 min)

fix: schemas/vegbien.sql: locationevent: added locationevent_place_visit_id index to facilitate joins to place_visit_id in the validations queries

bugfix: schemas/vegbien.sql: source_by_shortname(): documented that in some cases, it is actually a bad idea to use a nested SELECT, because this will prevent Postgres from using an index scan (causing an equally bad slowdown as not inlining in cases where a nested SELECT is required).

schemas/postgresql.conf: log_min_messages: dropped the verbosity back down to the default, to avoid clogging up the logs

schemas/vegbien.sql: locationevent: documented `VACUUM ANALYZE` runtime (20 min)

schemas/postgresql.conf: log_min_messages: show what autovacuum is doing