Web Pool needs cookieless persistence

You don't actually even need an iRule... You can use SSL session ID persistence which will (depending on the browser) persist all the same connections using the same SSL session to the same server. This can be enabled with the persist ssl setting on the virtual. You may also want to additionally use fallback persistence on the source IP to handle the transition some browsers go through while negotiating the SSL session. This is configured with the fallback persist srcaddr setting on the virtual.