caos.backends.pgsql: Fix handling property references in multipaths
caos.backends.pgsql: Fix handling property references in multipaths

The following query path:

 /-B-\
A    -@property
 \-C-/

Would cause pgsql transformer to crash.  While investigating this I
found a number of other issues with handling of the above paths.
Subject to further fixups.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb