Add stress test

This lets us trigger 'no unpinned buffers available' PostgreSQL errors.
Close portal after we've finished with it
Remember prepared statements

Never close prepared statements, and remember them and reuse them.
Essentially this is removing the use_cache option, and always using
it.
Invalidate PS cache on a DDL statement

This is because a change to the DB structure might invalidate prepared
statements.
Unknown type being sent with binary format code

Now sent with text format code. This solves problems with sending types
like tsvector which have text and binary representations.
Prepare for new release