Add set-default-search-path

Ambivalent about this because now the model code that uses it is not
idempotent (say they rebind *pgj-schema*).  But if we want them to use
'cat directly in a S-SQL query the only other way would be to trickily
substitute it for (:dot 'pgj-model 'booking)...
Wrap drop-backend! in a transaction

Do not complain if the backend schema does not exist.
Freshen up pp-json output
Change model interface ALL to GET-ALL

Consistency with DELETE-ALL