This adds support for queries in the form of

```
prepare stmt1; prepare stmt2; stmt3;
```

e.g.,

```
PREPARE p AS SELECT $1 AS x; EXECUTE p(42)
```

The session that stores the prepared statements is per
HTTP session. Since websockets currently close the connection
after a query, even websockets do not persist query state within
a session. But we may relax this in the future and not close the
socket after a single query when a websocket is opened.

Signed-off-by: Gerd Zellweger <mail@gerdzellweger.com>