caos.backends.pgsql: Use full path ids to identify join paths
caos.backends.pgsql: Use full path ids to identify join paths

Since inception, pgsql transformer used concept class as a key in join
path dictionary, so in joins involving multiple paths with the same
concept in them that could lead to an incorrect table being joined.
Fix this by using a full node path id as a join path key.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb