Improve PostgreSQL storage scheme for the with-path-denormalized flavor

Previous version was storing arrays of strings representing tuples for the
denormalized relations (dst and loc of the relation resp.). While that
simplified the check for duplicates, it turned out to be very inefficient
in terms of disk usage. The new version has two distinct lists if bigint
(ie. internal ids) for dst and loc resp. To check for duplicates the
lists should be zipped, and repeated tuples filtered.