Fix handling of nested polygons with `--update` mode

[rustprooflabs]

Details

This PR addresses #397 reported by @jhon100, and other issues exposed by investigating that bug report.

Overview

Running --update append was missing the pre/post steps used by the --replication mode to avoid similar errors. The prep step is now handled in one place for both. The post step is handled in one place each due to differing code paths.

Tasks

• [x] --update append update shouldn't require --force
• [x] Overlapping regions cause FK error in nested table even with --skip-nested
• [x] Clean out nested data when --update append to avoid FK error when nested data previously loaded
• [x] :construction: Improve documentation around --update mode
• [x] Test --replication to verify there wasn't a regression

[rustprooflabs]

Commands used for testing for reference:

export POSTGRES_USER=postgres
export POSTGRES_PASSWORD=mysecretpassword

docker stop pgosm && docker build -t rustprooflabs/pgosm-flex .

docker run --name pgosm -d --rm \
    -v ~/pgosm-data:/app/output \
    -v /etc/localtime:/etc/localtime:ro \
    -e POSTGRES_USER=$POSTGRES_USER \
    -e POSTGRES_PASSWORD=$POSTGRES_PASSWORD \
    -p 5433:5432 -d rustprooflabs/pgosm-flex

docker exec -it \
    pgosm python3 docker/pgosm_flex.py \
    --ram=8 \
    --region=north-america/us \
    --subregion=district-of-columbia \
    --update create

docker exec -it \
    pgosm python3 docker/pgosm_flex.py \
    --ram=8 \
    --region=north-america/us \
    --subregion=maryland \
    --update append