Backport capability to disable prepared statement caching to 0.8 line

[mp911de]

Given that pgbouncer seems a pretty common arrangement in production environments we should backport #223 to 0.8 to unblock users that want to use R2DBC Postgres with pgbouncer.

[x0a1b]

Can't wait for it!

[mp911de]

That's in place now.