change default of psql-livesync-threshold to 0

Currently, FKs are set with the 'NOT VALID' option. Consequently, dropping and restoring them does not take a long time. This means that the main reason for adding 'psql-livesync-threshold' no longer exists. The only reason the option is not removed altogether is that it allows for easy starting of the HAF in a LIVE state, which is crucial for tests.