Initial S-SQL forms to support JSON select queries

Simple example to demonstrate.
Give model parameters a proper get/set interface
Add FILTER and filter$

The are the interface and DB access functions respectively.
Surprisingly simple given the Postgres JSON support.
Simpler GIN indexing

This is a premature op (what isn't?) but the comments tell a good story
and it's pretty easy to revert...
Add booking.lisp fragments
Some sensible comments on filtering strategy
Very minor doco tweaks
Some lovely S-SQL compatible JSON based queries

Using the standard interface to make them on demand...
Progress more generic JDOC queries
In fact, S-SQL had nearly everything we needed!

A bit verbose but we can work on that.
Much simpler define-query form

Largely in vanilla S-SQL now.  Some additions to postmodern/prepare.lisp
to support a list of named parameters to prepared queries.
Some syntactic JSON sugar for define-query
Make isolation level settings unprepared queries

Postgres was moaning intermittently about this.  Frankly I don't
understand why it was intermittent but am not stopping to mope.
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
Remove with-db-schema
Remove crap from query.lisp
Rename FILTER to CONTAINS.  Add EXISTS.

These are pretty close to their Postgres progenitors.