Update to RSA bits

We used to have 2000 bit RSA keys, just to tickle software with not-so-standard sizes and see how it fared. But the habit of doubling RSA sizes (which is a bit steep, considering that it is closing in on exponential in bit sizes anyway) has overtaken us, in the form of a requirement in GnuTLS to have these funny ever-doubling sizes. Alas, TLS Pool follows that now.