Add 'ignore_nulls' option to row_to_json

Provide an option to skip NULL values in a row when generating a JSON
object from that row with row_to_json.  This can reduce the size of the
JSON object in cases where columns are NULL without really reducing the
information in the JSON object.

This also makes row_to_json into a single function with default values,
rather than having multiple functions.  In passing, change array_to_json
to also be a single function with default values (we don't add an
'ignore_nulls' option yet- it's not clear that there is a sensible
use-case there, and it hasn't been asked for in any case).

Pavel Stehule
Fix vacuumdb --analyze-in-stages --all order

When running vacuumdb --analyze-in-stages --all, it needs to run the
first stage across all databases before the second one, instead of
running all stages in a database before processing the next one.

Also respect the --quiet option with --analyze-in-stages.