Prepared statements: switch to use PQprepare / PQexecPrepared

[GitMensch]

Currently prepared statements are done manually. This seems to at least has the benefit of directly supporting cursor access to those (not sure if this would be possible "directly" otherwise), but has the downside of re-creating something libpg and the the DB has put in man-years of effort, not being able to see the prepared statements on the db side, ...

Doing this change would also fix #67 "for free".