ACE minor fixes

[pct960]

This PR introduces several minor improvements to ACE.

• If the cluster json file does not have a password specified, ACE will now attempt to read from the pgpass file.
• When a table does not exist on one of the nodes, ACE will now report that the table is missing instead of reporting that the table name is invalid.
• Earlier, the ACE user had to be the owner of the table to perform table-diff. This was due to a query that was carried over from the very first version of ACE that needed the ACE user to be the table owner. This query has now been updated, and a user with sufficient permissions can perform table-diff, without having to be the table owner.
• Added application_name=ACE for log auditing purposes.
• In the case of a bytea datatype of a column, ACE will convert it to a hex representation before producing the diff file. It will also short-circuit out of table-diff if the bytea column is too large.
• All ACE modules will now perform a privilege check before attempting to execute any operation. Except table-diff, all modules need SELECT. table-repair needs SELECT, INSERT, UPDATE and optionally DELETE.
• Multiprocessing worker connection pool lifecycle is now properly handled to avoid unexpected EOF errors.
• Streamlined sanity checks so that multiple passes on the connection list are no longer needed.
• Removed redundant checks in table-rerun.
• table-rerun now correctly requests for necessary privileges when behavior is hostdb, and also properly handles mixed-case table and column names.