pgimport enhancements

[turicas]

• It's very useful to run VACUUM ANALYZE tablename; just after importing data, so we could have an option on pgimport to do it (function and CLI).
• The performance of pgimport could be better if vacuum for the imported table is disabled before import (ALTER TABLE tablename SET (autovacuum_enabled = off);) and then (after import) re-enabled. This could be the default behaviour but may be an option to disable it (could be useful in cases where we're importing data into an existing table).