Support prepared statements / statement cache when loading array associations

[DanielHeath]

I've figured out the bug which was preventing the statement cache being used with array associations.

Unfortunately, I'm unlikely to get around to opening a PR for it in the near future.

My fork is very different (I've stripped out parts of the library I'm not using, to reduce the chance of conflicts with core rails), but cb76495 over on https://github.com/radiopaedia/torque-postgresql/tree/just_has_one should point you in the right direction.

[DanielHeath]

Looks like there's still an issue where it'll sometimes create too many bind params, actually - possibly not as useful as I'd hoped.